Adds a number of user management/ACL validaiton/command execution functions to improve a
Redis module's ability to enforce ACLs correctly and easily.
* RM_SetContextUser - sets a RedisModuleUser on the context, which RM_Call will use to both
validate ACLs (if requested and set) as well as assign to the client so that scripts executed via
RM_Call will have proper ACL validation.
* RM_SetModuleUserACLString - Enables one to pass an entire ACL string, not just a single OP
and have it applied to the user
* RM_GetModuleUserACLString - returns a stringified version of the user's ACL (same format as dump
and list). Contains an optimization to cache the stringified version until the underlying ACL is modified.
* Slightly re-purpose the "C" flag to RM_Call from just being about ACL check before calling the
command, to actually running the command with the right user, so that it also affects commands
inside EVAL scripts. see #11231
