Support Postgres composite types [DBZ-994]

Migrated from [DBZ-994](https://issues.redhat.com/browse/DBZ-994)

Postgres [Composite types|https://www.postgresql.org/docs/current/rowtypes.html] allow structured types to be used as column types:

{code}
CREATE TYPE phone AS (
area text,
numb integer
);

ALTER TABLE customers ADD phone_no phone;
{code}

Debezium should capturing values to such columns. Some open questions:

* Are changes to the logical decoding plug-ins needed?
* How to obtain information about the columns contained in a composite type?
* How to represent columns of composite types in emitted CDC messages? Could be done as an equivalent struct or just by adding the contained columns directly to the before/after structs. The latter increases compatibility with consumers, but it loses information about the containing composite type. This might be mitigated by adding the composite type name as a schema parameter of the schemas for the contained columns. That'd be my preference.

Start with the PostgreSQL composite-type example in the issue and investigate the logical decoding plug-ins, how composite-column metadata can be obtained, and how values should appear in emitted CDC messages. Done requires resolving those representation and compatibility questions and supporting captured values for composite-type columns.

Written by the indexing model from the issue text.