Redis RedisModule_KeyAtPos and COMMAND GETKEYS should be able to handle key-spec flags.

The first one is needed for proper ACL key permissions support #9974

When we do that we should also expose the variable_flags flag we introduced in #10148 to both modules (commandKeySpecsFlagsFromString) and clients (addReplyFlagsForKeyArgs), and document them.

Comment From: zuiderkwast

I guess we can't change the signature void RM_KeyAtPos(RedisModuleCtx *ctx, int pos).

Do we need to add another API like RM_KeyAtPosWithFlags?