This is needed in order to ease the deployment of functions for ephemeral cases, where user needs to spin up a server with functions pre-loaded.
See #9899 (comment) and following comments.

Details:

• Added --functions-rdb option to redis-cli.
• Functions only rdb via REPLCONF rdb-filter-only functions. This is a placeholder for a space separated inclusion filter for the RDB. In the future can be REPLCONF rdb-filter-only "functions db:3 key-patten:user*" and a complementing rdb-filter-exclude REPLCONF can also be added.
• Handle "slave requirements" specification to RDB saving code so we can use the same RDB when different slaves express the same requirements (like functions-only) and not share the RDB when their requirements differ. This is currently just a flags int, but can be extended to a more complex structure with various filter fields.
• make sure to support filters only in diskless replication mode (not to override the persistence file), we do that by forcing diskless (even if disabled by config)

other changes:

• some refactoring in rdb.c (extract portion of a big function to a sub-function)
• rdb_key_save_delay used in AOFRW too

TODO:

• tests.
• handles TODOs in the PR code.