Adds pub/sub channel patterns to ACL (redis#7993)

Fixes redis#7923.

This PR appropriates the special `&` symbol (because `@` and `*` are taken),
followed by a literal value or pattern for describing the Pub/Sub patterns that
an ACL user can interact with. It is similar to the existing key patterns
mechanism in function (additive) and implementation (copy-pasta). It also adds
the allchannels and resetchannels ACL keywords, naturally.

The default user is given allchannels permissions, whereas new users get
whatever is defined by the acl-pubsub-default configuration directive. For
backward compatibility in 6.2, the default of this directive is allchannels but
this is likely to be changed to resetchannels in the next major version for
stronger default security settings.

Unless allchannels is set for the user, channel access permissions are checked
as follows :
* Calls to both PUBLISH and SUBSCRIBE will fail unless a pattern matching the
  argumentative channel name(s) exists for the user.
* Calls to PSUBSCRIBE will fail unless the pattern(s) provided as an argument
  literally exist(s) in the user's list.

Such failures are logged to the ACL log.

Runtime changes to channel permissions for a user with existing subscribing
clients cause said clients to disconnect unless the new permissions permit the
connections to continue. Note, however, that PSUBSCRIBErs' patterns are matched
literally, so given the change bar:* -> b*, pattern subscribers to bar:* will be
disconnected.

Notes/questions:
* UNSUBSCRIBE, PUNSUBSCRIBE and PUBSUB remain unprotected due to lack of reasons
  for touching them.

Author: itamarhaber

redis.conf

@@ -747,6 +747,11 @@ replica-priority 100
 #               It is possible to specify multiple patterns.
 #  allkeys      Alias for ~*
 #  resetkeys    Flush the list of allowed keys patterns.
+#  &<pattern>   Add a glob-style pattern of Pub/Sub channels that can be
+#               accessed by the user. It is possible to specify multiple channel
+#               patterns.
+#  allchannels  Alias for &*
+#  resetchannels            Flush the list of allowed channel patterns.
 #  ><password>  Add this password to the list of valid password for the user.
 #               For example >mypass will add "mypass" to the list.
 #               This directive clears the "nopass" flag (see later).
@@ -820,6 +825,27 @@ acllog-max-len 128
 #
 # requirepass foobared
 
+# New users are initialized with restrictive permissions by default, via the
+# equivalent of this ACL rule 'off resetkeys -@all'. Starting with Redis 6.2, it
+# is possible to manage access to Pub/Sub channels with ACL rules as well. The
+# default Pub/Sub channels permission if new users is controlled by the 
+# acl-pubsub-default configuration directive, which accepts one of these values:
+#
+# allchannels: grants access to all Pub/Sub channels
+# resetchannels: revokes access to all Pub/Sub channels
+#
+# To ensure backward compatibility while upgrading Redis 6.0, acl-pubsub-default
+# defaults to the 'allchannels' permission.
+#
+# Future compatibility note: it is very likely that in a future version of Redis
+# the directive's default of 'allchannels' will be changed to 'resetchannels' in
+# order to provide better out-of-the-box Pub/Sub security. Therefore, it is
+# recommended that you explicitly define Pub/Sub permissions for all users
+# rather then rely on implicit default values. Once you've set explicit
+# Pub/Sub for all exisitn users, you should uncomment the following line.
+#
+# acl-pubsub-default resetchannels
+
 # Command renaming (DEPRECATED).
 #
 # ------------------------------------------------------------------------