feat: value renderer for timestamp protos (#12860)

## Description

Closes: #12709



Part of Sign Mode Textual (ADR 050) implementation.

Renders Timestamp messages as RFC 3339 (simplified ISO 8601).

---

### Author Checklist

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- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [x] added `!` to the type prefix if API or client breaking change
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- [ ] added a changelog entry to `CHANGELOG.md`
- [x] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [x] updated the relevant documentation or specification
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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
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- [ ] manually tested (if applicable)

docs/architecture/adr-050-sign-mode-textual-annex1.md

@@ -203,7 +203,20 @@ Object: /cosmos.gov.v1.Vote
 
 ### `google.protobuf.Timestamp`
 
-Rendered as either ISO8601 (`2021-01-01T12:00:00Z`).
+Rendered using [RFC 3339](https://www.rfc-editor.org/rfc/rfc3339) (a
+simplification of ISO 8601), which is the current recommendation for portable
+time values. The rendering always uses "Z" (UTC) as the timezone. It uses only
+the necessary fractional digits of a second, omitting the fractional part
+entirely if the timestamp has no fractional seconds. (The resulting timestamps
+are not automatically sortable by standard lexicographic order, but we favor
+the legibility of the shorter string.)
+
+#### Examples
+
+The timestamp with 1136214245 seconds and 700000000 nanoseconds is rendered
+as `2006-01-02T15:04:05.7Z`.
+The timestamp with 1136214245 seconds and zero nanoseconds is rendered
+as `2006-01-02T15:04:05Z`.
 
 ### `google.protobuf.Duration` (TODO)