Git tags for minor versions

There's currently two Git tags defined in bazelbuild/remote-apis:
* Tag `v2.0.0` is commit e77c4eb2ca4804854daf6115405b557a894eca9f and is (hopefully) what implementations mean by `SemVer { major: 2 }`.
* Tag `4ac0011` is commit 4ac001182e50d404fa790fb4a321daca089fe18c and is of unknown meaning.

The comments in `remote_execution.proto` refer to changes in minor versions v2.1, v2.2, and v2.3 which are not tagged. This makes it difficult to determine which behavior a conforming implementation should have at each version.

Given that there's now multiple APIs specified, and multiple versions of the Remote Execution API, would it be possible to add additional tags? For example:
* `execution/v2.0`, `execution/v2.1`, `execution/v2.2`, and `execution/v2.3` that can be used as stable references for a given version of `remote_execution.proto`
* `asset/v1.0` and `logstream/v1.0` for similar purposes in the `remote_asset.proto` and `remote_logstream.proto` files.

Review the existing tags and the version comments in remote_execution.proto, remote_asset.proto, and remote_logstream.proto. Check the repository's release and tagging conventions first, then determine which commits correspond to the requested API versions. Done means stable tags exist for the specified execution, asset, and logstream versions and clearly identify their referenced API definitions.

Written by the indexing model from the issue text.