Custom WAL Resource Managers.

Allow extensions to specify a new custom resource manager (rmgr),
which allows specialized WAL. This is meant to be used by a Table
Access Method or Index Access Method.

Prior to this commit, only Generic WAL was available, which offers
support for recovery and physical replication but not logical
replication.

Reviewed-by: Julien Rouhaud, Bharath Rupireddy, Andres Freund
Discussion: https://postgr.es/m/ed1fb2e22d15d3563ae0eb610f7b61bb15999c0a.camel%40j-davis.com

doc/src/sgml/ref/pg_waldump.sgml

@@ -173,6 +173,14 @@ PostgreSQL documentation
         If <literal>list</literal> is passed as name, print a list of valid resource manager
         names, and exit.
        </para>
+       <para>
+        Extensions may define custom resource managers, but pg_waldump does
+        not load the extension module and therefore does not recognize custom
+        resource managers by name. Instead, you can specify the custom
+        resource managers as <literal>custom###</literal> where
+        "<literal>###</literal>" is the three-digit resource manager ID. Names
+        of this form will always be considered valid.
+       </para>
       </listitem>
      </varlistentry>