Enable `EnumCodec.builder` to handle `Enum` with different schema and same name.

[k163377]

Feature Request

SSIA

Is your feature request related to a problem? Please describe

EnumCodec.builder().withEnum will fail if a Enum with a different schema and the same name is registered.

Caused by: java.lang.IllegalArgumentException: Builder contains already a mapping for Postgres type ...
	at io.r2dbc.postgresql.codec.EnumCodec$Builder.withEnum(EnumCodec.java:180)

Describe the solution you'd like

I think there should be an option to specify the schema name as well.

Describe alternatives you've considered

I couldn't think of any.
If you have any ideas on how to do this, I would be happy to get support.

Teachability, Documentation, Adoption, Migration Strategy