docs(adr): Update protobuf guidelines with Since (#10672)

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## Description

I've been telling people to add this `Since:` comment in some PRs, but noticed it's not documented anywhere. I updated the relevant PR from the conclusions of TX working group calls with @aaronc, @webmaster128 and myself.

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---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [ ] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] added `!` to the type prefix if API or client breaking change
- [ ] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [ ] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [ ] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
- [ ] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

docs/architecture/adr-044-protobuf-updates-guidelines.md

@@ -3,6 +3,7 @@
 ## Changelog
 
 - 28.06.2021: Initial Draft
+- 02.12.2021: Add `Since:` comment for new fields
 
 ## Status
 
@@ -47,10 +48,38 @@ For this reason, module developers MUST NOT add new fields to existing `Msg`s.
 
 It is worth mentioning that this does not limit adding fields to a `Msg`, but also to all nested structs and `Any`s inside a `Msg`.
 
-#### 2. Non-`Msg`-related Protobuf definitions MAY have new fields
+#### 2. Non-`Msg`-related Protobuf definitions MAY have new fields, but MUST add a `Since:` comment
 
 On the other hand, module developers MAY add new fields to Protobuf definitions related to the `Query` service or to objects which are saved in the store. This recommendation follows the Protobuf specification, but is added in this document for clarity.
 
+The SDK requires the Protobuf comment of the new field to contain one line with the following format:
+
+```protobuf
+// Since: cosmos-sdk <version>{, <version>...}
+```
+
+Where each `version` denotes a minor ("0.45") or patch ("0.44.5") version from which the field is available. This will greatly help client libraries, who can optionally use reflection or custom code generation to show/hide these fields depending on the targetted node version.
+
+As examples, the following comments are valid:
+
+```protobuf
+// Since: cosmos-sdk 0.44
+
+// Since: cosmos-sdk 0.42.11, 0.44.5
+```
+
+and the following ones are NOT valid:
+
+```protobuf
+// Since cosmos-sdk v0.44
+
+// since: cosmos-sdk 0.44
+
+// Since: cosmos-sdk 0.42.11 0.44.5
+
+// Since: Cosmos SDK 0.42.11, 0.44.5
+```
+
 #### 3. Fields MAY be marked as `deprecated`, and nodes MAY implement a protocol-breaking change for handling these fields
 
 Protobuf supports the [`deprecated` field option](https://developers.google.com/protocol-buffers/docs/proto#options), and this option MAY be used on any field, including `Msg` fields. If a node handles a Protobuf message with a non-empty deprecated field, the node MAY change its behavior upon processing it, even in a protocol-breaking way. When possible, the node MUST handle backwards compatibility without breaking the consensus (unless we increment the proto version).
@@ -70,7 +99,7 @@ TODO, needs architecture review. Some topics:
 
 - Bumping versions frequency
 - When bumping versions, should the Cosmos SDK support both versions?
-    - i.e. v1beta1 -> v1, should we have two folders in the Cosmos SDK, and handlers for both versions?
+  - i.e. v1beta1 -> v1, should we have two folders in the Cosmos SDK, and handlers for both versions?
 - mention ADR-023 Protobuf naming
 
 ## Consequences