Precompile SELECT set_config to SET

[aljazerzen]

This is an optimistic PR that might not be reasonable to fully implement.

I've added a pre-compilation step that turns SELECT set_config into SET commands.

The problem now is that SELECT set_config:

• returns CommandComplete('SET'), but it should return CommandComplete('') (this can be solved easily),
• returns no result, but it should return a single row with a single column that is the value of the setting that was just set (this would be harder to solve),

We don't have a framework for injecting result tuples for SET commands that would work in general (for frontend-only actions and the normal ones). So I'm not sure if I should continue here.

The pro is that we would then support all basic set_config usages (top-level, without complicated SELECT clauses).

The cons is that is needed only now, for solving #7804, where we need to just ignore a pg setting.

[aljazerzen]

The conclusion here is that:

• we need to get pg_dump working now, so let's just add another workaround instead of this PR,
• I will come back to this later, hopefully after pgcon.pxd will be rewritten in Rust.