Add support for `flags` parameter of `redis.register_function`

### Search before asking

https://redis.io/docs/interact/programmability/functions-intro/#function-flags

### Motivation

compatibility

### Solution

_No response_

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

Start with the Redis documentation linked in the issue, especially the function-flags section, then trace Kvrocks' implementation of the redis.register_function entry point and its existing tests. Add compatibility for the flags parameter and verify the supported flag behavior with focused tests.

Written by the indexing model from the issue text.