Add transforms feature

This provides a mechanism for specifying conversions between SQL data
types and procedural languages.  As examples, there are transforms
for hstore and ltree for PL/Perl and PL/Python.

reviews by Pavel Stěhule and Andres Freund

doc/src/sgml/reference.sgml

@@ -111,6 +111,7 @@
    &createTSDictionary;
    &createTSParser;
    &createTSTemplate;
+   &createTransform;
    &createTrigger;
    &createType;
    &createUser;
@@ -152,6 +153,7 @@
    &dropTSDictionary;
    &dropTSParser;
    &dropTSTemplate;
+   &dropTransform;
    &dropTrigger;
    &dropType;
    &dropUser;