feat(proto)!: replace pub_key with pub_key_type (#2890)

and `pub_key_bytes` in `privval.PubKeyResponse`, `types.Validator` and
`types.SimpleValidator`.

Closes #2878
Follow-up to #2843
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---

#### PR checklist

- [x] Tests written/updated
- [x] Changelog entry added in `.changelog` (we use
[unclog](https://github.com/informalsystems/unclog) to manage our
changelog)
- [ ] Updated relevant documentation (`docs/` or `spec/`) and code
comments
- [x] Title follows the [Conventional
Commits](https://www.conventionalcommits.org/en/v1.0.0/) spec

---------

Co-authored-by: Sergio Mena <sergio@informal.systems>
(cherry picked from commit d342b3a)

.changelog/unreleased/breaking-changes/2878-proto-privval-get-pubkey-response-public-key.md

@@ -0,0 +1,2 @@
+- [`proto/privval`]  Replace `pub_key` with `pub_key_type` and `pub_key_bytes` in
+  `PubKeyResponse` ([\#2878](https://github.com/cometbft/cometbft/issues/2878))

.changelog/unreleased/breaking-changes/2878-proto-types-validator-public-key.md

@@ -0,0 +1,2 @@
+- [`proto/types`] Replace `pub_key` with `pub_key_type` and `pub_key_bytes` in
+  `Validator` and `SimpleValidator` ([\#2878](https://github.com/cometbft/cometbft/issues/2878))