Add pg_lsn postgres data type

[arpad-m]

A pg_lsn encodes an offset into the Postgres Write Ahead Log (WAL). It is a native type of Postgres.

[Ten0]

Thanks a lot for your contribution!

I have no strong opinion on whether this should live in Diesel (I'd say I'm 80% sure that others will say yes, but unless I'm mistaken this could alternatively be implemented in any given schema crate and I doubt many people use this, and this seems to add non trivial code) so I'll let others voice their opinions there.

That being said If we do indeed merge this I think I notice a few things that we might want to address before merging.

Thanks again! 🤗

[weiznich]

Thanks for contribution. This looks fine to me beside that minor remark about the commented out code.

[arpad-m]

@weiznich @Ten0 I have addressed your points. ready for review again.

[Ten0]

Thanks! 🤗