PostgreSQL JDBC Struct/Array Support

Currently, the composite types defined in PostgreSQL (https://www.postgresql.org/docs/current/rowtypes.html#ROWTYPES) are not exposed through the pgjdbc driver. This proposal aims to enable such functionality by implementing the methods defined in the JDBC API. It will allow a user to both retrieve a Struct from the result-set or use it for insertion or updates. Since a row can also hold an array of composite types, properly handling these arrays during reading / writing will also be taken in consideration.