[Streaming Replication 5th] add connection configuration for Streaming Replication Protocol

[osaxma]

This change simply allows running PostgreSQLConnection in replication mode in the following manner:

  final conn = PostgreSQLConnection(
    'localhost',
    5432,
    'postgres',
    username: 'postgres',
    password: 'postgres',
    replicationMode: ReplicationMode.logical,
  );

Tests were added which shows the use of useSimpleQueryProtocol (turning that off will make the test fail)

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[osaxma]

I just realized logicalDecodingPlugin isn't necessary since this is defined by the user when they start the replication process (i.e. when calling CREATE_REPLICATION_SLOT)...

I'll remove it since it doesn't make sense and then turn this as Ready for Review.