fix(redis): add redis_server_name configuration

[Arjen10]

Description

Implemented two changes:

1. Added the redis_server_name configuration for Redis connections.
2. Added redis_server_name to the Redis connection pool.

Which issue(s) this PR fixes:

Fixes #13926

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[Arjen10]

This PR only fixes TLS SNI for standalone Redis (policy: redis), which is the path described in #13926.

policy: redis-cluster uses a different client. Cluster SNI is out of scope here and should be handled in a follow-up PR.

[Arjen10]

Closing this as #13938 has been merged for #13926.

The first commit on that PR came from this branch. I would have preferred the follow-ups to land here, but there is nothing left to review on this side.