Add support of custom codec registration

[rchigvintsev]

Feature Request

Problem

I use simple custom codec to map Java enumeration name to VARCHAR in case of PostgreSQL database. I register it with implementation of io.r2dbc.postgresql.extension.CodecRegistrar. When I use H2 in-memory database to perform integration testing it is not possible to register the similar codec as easy as for PostgresSQL.

Solution

Maybe it would be nice to migrate extension mechanism from r2dbc-postgresql repository to this project.

[alpeshvas]

Any plan on this?

[alpeshvas]

@lukaseder Could we change visibility (currently it's package protected) of AbstractCodec ? So that we can implement custom codecs and provide them through H2ConnectionFactoryProvider. Adding registrar support could take time.

[lukaseder]

I'm the wrong guy to ask here

[alpeshvas]

I'm the wrong guy to ask here
Okay, tagging original authors of the file

@nebhale @gregturn