spec(proto): add cometbft.privval.v1beta2 proto package (cometbft#2529

)

The package is added to represent the privval protocol in CometBFT
0.38.x, since the added field in `SignVoteRequest` (in a string of PRs
ending in cometbft#2522) and removal of the (unused) `Errors` enum make v1
different.

---

#### PR checklist

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

.changelog/unreleased/improvements/2522-privval-skip-extension-signing.md

@@ -1,2 +1,5 @@
 - `[proto]` Add `skip_extension_signing` field to the `SignVoteRequest` message
-  in privval ([\#2522](https://github.com/cometbft/cometbft/pull/2522))
+  in `cometbft.privval.v1` ([\#2522](https://github.com/cometbft/cometbft/pull/2522)).
+  The `cometbft.privval.v1beta2` package is added to capture the protocol as it was
+  released in CometBFT 0.38.x
+  ([\#2529](https://github.com/cometbft/cometbft/pull/2529)).