Add support for plain SQL TableElement and CustomTableElement implementations

[lukaseder]

Users who wish to extend jOOQ's Constraint support with custom clauses can currently not add those clauses to generated DDL. They might resort to implementing internal API as can be seen here, but that's brittle:
https://groups.google.com/g/jooq-user/c/hywUqufzUmg

We could offer the construction of plain SQL TableElement and CustomTableElement implementations instead, where users can control the SQL generation themselves. This would also allow for adding arbitrary column specifications that we don't (yet) support.