[redis_proxy] Add support for UNWATCH

[dbarbosapn]

This PR adds support for the UNWATCH command in the Redis proxy.
Because of the lack of UNWATCH support, transactions are not well supported by clients such as StackExchange.Redis, since conditions require UNWATCH support.

The Challenge
The challenge with implementing UNWATCH in Envoy is knowing which server to send the UNWATCH command to.
This was solved in this PR by considering WATCH a Transaction Command. The behavior is explained below.

Expected Behavior

1. WATCH as a transaction command.
   Making WATCH a transaction command means it will be handled by the transaction handler, not the simple request handler. There, we set the transaction key in advance, without starting the transaction. This means we will have state of the key where the latest WATCH was sent to.

2. UNWATCH after WATCH
   If an UNWATCH is sent right after a WATCH, we send it to the server of the transaction key that was set. After that, we reset the key. Covered by test: ✅

3. UNWATCH without key
   Sending an UNWATCH without previous key being set, will have a direct proxy response of +OK\r\n. Covered by test: ✅

4. UNWATCH after MULTI (no key)
   Sending an UNWATCH without a key but within a transaction, will also be a no-op in the proxy. But the response this time is +QUEUED\r\n, to match Redis behavior. Covered by test: ✅

5. UNWATCH with key
   Sending an UNWATCH with a transaction key set (either by WATCH or a simple command within an active transaction) will send the command to the server of such hash key. Covered by test: ✅

6. MULTI after WATCH
   Since WATCH now sets the transaction key in advance, in this scenario we no longer queue the MULTI command, and we send it directly upstream, based on the key set by the WATCH command. Covered by test: ✅

7. MULTI after UNWATCH
   If an UNWATCH is sent after a WATCH, the transaction key is reset, and the MULTI command is queued. This is the existing behavior in the proxy. Covered by test: ✅

[dbarbosapn]

cc @mattklein123 @weisisea shorter one this time 😄

[mgravell]

@dbarbosapn if this is accepted, please make sure to let me know so I can update the client library's data

[mattklein123]

Sorry needs main merge.

/wait

[dbarbosapn]

Sorry needs main merge.

/wait

Done! Thank you!! 🙇