Remove the old message PublicKey proto oneof (#7390)

* Remove unused PublicKey type and update docs

* Update wording

* Update proto/cosmos/base/crypto/v1beta1/crypto.proto

Co-authored-by: Amaury Martiny <amaury.martiny@protonmail.com>

* Edit docs

* Move crypto.proto to multisig

* Proto linting

* Update docs/architecture/adr-020-protobuf-transaction-encoding.md

Co-authored-by: Amaury Martiny <amaury.martiny@protonmail.com>

* Update wording for Public Key Encoding

Co-authored-by: Amaury Martiny <amaury.martiny@protonmail.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
Co-authored-by: Robert Zaremba <robert@zaremba.ch>

docs/architecture/adr-020-protobuf-transaction-encoding.md

@@ -10,6 +10,7 @@
 - 2020 June 08: Store `TxBody` and `AuthInfo` as bytes in `SignDoc`; Document `TxRaw` as broadcast and storage type.
 - 2020 August 07: Use ADR 027 for serializing `SignDoc`.
 - 2020 August 19: Move sequence field from `SignDoc` to `SignerInfo`.
+- 2020 September 25: Remove `PublicKey` type in favor of `secp256k1.PubKey`, `ed25519.PubKey` and `multisig.LegacyAminoPubKey`.
 
 ## Status
 
@@ -284,27 +285,21 @@ and `FileDescriptor`s and returns a boolean result.
 
 ### Public Key Encoding
 
-Public keys in the Cosmos SDK implement Tendermint's `crypto.PubKey` interface,
-so a natural solution might be to use `Any` as we are doing for other interfaces.
-There are, however, a limited number of public keys in existence and new ones
-aren't created overnight. The proposed solution is to use a `oneof` that:
-
-- attempts to catalog all known key types even if a given app can't use them all
-- has an `Any` member that can be used when a key type isn't present in the `oneof`
+Public keys in the Cosmos SDK implement Tendermint's `crypto.PubKey` interface.
+We propose to use `Any` for protobuf encoding as we are doing with other interfaces (e.g. in `BaseAccount` `PubKey` or `SignerInfo` `PublicKey`).
+Following public keys are implemented: secp256k1, ed25519 and multisignature.
 
 Ex:
 
 ```proto
-message PublicKey {
-    oneof sum {
-        bytes secp256k1 = 1;
-        bytes ed25519 = 2;
-        ...
-        google.protobuf.Any any_pubkey = 15;
-    }
+message PubKey {
+    bytes key = 1;
 }
 ```
 
+`multisig.LegacyAminoPubKey` has an array of `Any`'s member to support any
+protobuf public key type.
+
 Apps should only attempt to handle a registered set of public keys that they
 have tested. The provided signature verification ante handler decorators will
 enforce this.