Move dbt server (RPC) into new codebase

[jtcohen6]

We're rethinking the future of the dbt server, with the following goals:

• stateless
• faster to initialize
• faster multiprocessing of async requests (may require writing in a language other than python)
• requiring less overhead
• loosely coupled with specific dbt versions
• eventually, supporting a more performant event/interaction protocol in addition to / instead of JSON-RPC

To that end, we'd like to move the logic that currently defines dbt rpc into a new, separate codebase (proposed name: dbt-core-api). The new package would import dbt-core, but it would be distributed and versioned separately from dbt-core, for two main reasons:

• The server should be able to abstract over several dbt versions, and add in backwards-compatible server-only functionality
• The server should not be major-version 1 at the same time as dbt-core==1.0.0

[kwigley]

dupe of #3675