add parser interface in engine

[jennifersp]

This PR creates sql.Parser interface. This interface is defined in the engine and it should be used rather than using mysql parser directly.
Added GlobalParser variable to expose Doltgres parser for parsing view definition for now. It can also be used in places that needs doltgres-specific syntax parsing.

[zachmu]

Parser interface looks good, putting it on the context looks bad.

It can go on the catalog if it needs to, but it probably shouldn't. The main place that needs it is the plan builder, it should be a field there.

[zachmu]

Mostly looks good, just a couple pieces of feedback