[CT-706] Consolidate data type methods in adapter interface

[jtcohen6]

Placeholder ticket for further investigation down the line, prompted by larger initiative for moving dbt_utils functionality into dbt-core + plugins.

There are three ways we encode data types today, and make them available to users / adapter maintainers:

• dbt_utils.type_* macros
• Column class type translation
• agate type conversion methods, which translate agate's inferred types into each database's actual type system

I'd like to see us reconcile and consolidate those data type definitions as much as possible:

• One standard set of shared types, clearly enumerated — we sort of already have this in the form of agate types for seeds (text, number, boolean, datetime, date, time)
• One single mapping function, translate_type, extensible/overridable for each adapter. This would translate between our standard types, and each database's type system.

As a first pass, I'm working to rewrite the dbt_utils.type_* macros to use Column.translate_type instead: dbt-labs/dbt-utils#598, dbt-labs/dbt-utils#586

[nathaniel-may]

This has the potential to be the building blocks of something much bigger. Why not translate function types as well to compose sql together? This is a good first step, but we could have a discussion on what the natural expansion of this looks like and see if it would be a good fit for a future adapter interface.