Cache Capability to indicate that CAS is read-only

We need to signal to the client that in some configurations, the CAS will only accept read requests. All write requests will result in errors.

Given that we are currently advertising

```protobuf
// Describes the server/instance capabilities for updating the action cache.
message ActionCacheUpdateCapabilities {
bool update_enabled = 1;
}
```

in `CacheCapabilities` proto. I was thinking that we could simply add another identical field for CASUpdateCapabilities.

However, such a field would default to `false`, which is troublesome for backward compatibility. And doing `bool is_readonly = 1;` feels out of spec. Looking for feedback on this design.

Review the CacheCapabilities proto and the existing ActionCacheUpdateCapabilities message described in the issue. Determine how CAS read-only capability signaling can preserve backward compatibility, then document the agreed field design and update the API definition accordingly.

Written by the indexing model from the issue text.