Add support for SPI Parameters

[mp911de]

See r2dbc/r2dbc-spi#160.

We should extend our PostgresqlObjectId to implement Type so that it can be used together with Parameters.in(…) (e.g. Parameters.in(PostgresqlObjectId.CIRCLE)) and we should propagate the desired data type to the encoded value. We also should allow dynamic type look up:

PostgresTypes types = PostgresTypes.from(this.connection);
PostgresTypes.PostgresType type = types.lookupType("my_enum").block();

…
Parameters.in(type, "HELLO")

so that runtime types can be used with our codecs.