Add support for PostgreSQL Domain

When a column is defined using a postgres DOMAIN the underlying type is not being outputted from either wal2json plugin or protobuff. Instead the domain name is given as the type, which causes Debezium to skip processing this field when include.unknown.datatypes is set as false.

Example:

Given schema:

create domain sf_id as varchar(18);

create table payment
(
 id serial not null constraint payment_pkey primary key,
 salesforce_id sf_id,
);

Debezium will complain that sf_id is an unknown type, and filter out the column.

Expected: Debezium will treat this column as varchar(18)

Solution can come either from plugins side, to allow the option to produce the underlying type instead of domain name, or from postgresql-connector - to get domain to underlying type mapping from DB.

Feature request created for wal2json: https://github.com/eulerto/wal2json/issues/85
Google group thread: https://groups.google.com/forum/#!topic/debezium/6Kp7HMfWLPY