Moving pg_catalog to a separated module

[sunng87]

I'm also using the pg_catalog compatibility features in greptimedb. But it fetches pgwire and tokio, which is not directly used if pgwire handler are implemented separately.

So the idea is to move pg_catalog and sql module to a new module that contains only the pg_catalog compatibility features, with only dependencies to datafusion.

I will wait for #178 resolved to do the refactoring.

[mjgarton]

This idea seems sensible. I should mention that I don't know yet when I will have time to continue with #178 so maybe it's best not to wait if you don't want to be blocked.

[sunng87]

@mjgarton No worry, to be accurate I don't have wait for whole #178 being resolved, just to make sure if you don't have any pull requests that is work in progress because it will introduce conflicts.

Overall I'm not blocked by this issue too. It's not in urgent. I already have a branch to work with greptimedb.