Reduce lock levels of some trigger DDL and add FKs

Reduce lock levels to ShareRowExclusive for the following SQL
 CREATE TRIGGER (but not DROP or ALTER)
 ALTER TABLE ENABLE TRIGGER
 ALTER TABLE DISABLE TRIGGER
 ALTER TABLE … ADD CONSTRAINT FOREIGN KEY

Original work by Simon Riggs, extracted and refreshed by Andreas Karlsson
New test cases added by Andreas Karlsson
Reviewed by Noah Misch, Andres Freund, Michael Paquier and Simon Riggs

doc/src/sgml/ref/alter_table.sgml

@@ -406,6 +406,9 @@ ALTER TABLE ALL IN TABLESPACE <replaceable class="PARAMETER">name</replaceable>
       mode, and triggers configured as <literal>ENABLE ALWAYS</literal> will
       fire regardless of the current replication mode.
      </para>
+     <para>
+      This command acquires a <literal>SHARE ROW EXCLUSIVE</literal> lock.
+     </para>
     </listitem>
    </varlistentry>