Added the ExtendedType interface

[Hydrocharged]

This adds the ExtendedType interface, which is used within DoltgreSQL to implement PostgreSQL types, as well as in Dolt to properly handle the new type and value serialization.

Related PRs:

• Added storage capabilities for GMS Extended Types dolt#7457
• Added BOOLEAN and BOOLEAN[] types doltgresql#120

[zachmu]

LGTM

[max-hoffman]

One weird thing is how this goes against our current type organizational patterns. There is a world where GMS doesn't know anything about type implementations, and Dolt/Postgres initializes an engine with types alongside storage specific serialization and comparison code. I think that could be better in some ways, because the engine wouldn't have to hop back and forth between the prolly and sql layers as often. Better perf and probably less type indirection. That's kind of what ExtendedType does. On the other hand, all of the other types explicitly separate logical and storage layer details. And that's something that doesn't seem much harder than the existing dolt changes outside of typeinfo and types which are almost deprecated. GMS could have an extended type for behavioral details, and Dolt could have a similar thing for extending serialization patterns for the same ExtendedTypes. For me it's weird to support both patterns at the same time. That plus skipping semantic resolution are my biggest concerns.