use JSONSchema $schema for dbt contracts (via dataclasses + jsonschema-dataclasses)

[beckjake]

Feature

dbt should use the $schema and $ref tags of JSONSchema instead of the current deep_merge and manually-referencing dicts.

Feature description

We should convert our json schema contracts to have $schema values pointing at publicly accessible URLs, use the $ref feature to link behavior of our internal JSON schemas, and modify our APIObject and its descendants to take advantage of it.

We can start by specifying just the outermost "layers" of the schema and add the references as we go along. We should do something like iglu uses and define a decent versioned self-describing format. An example iglu schema is here

We should support namespacing, and give dbt-core a separate namespace from any plugins (like adapters!) or dbt-cloud.

We also will want to specify credential contracts on a per-adapter basis, and adapter-specific configurations at some point (currently we just allow anything).

Serialization should include the schema value so we can try to do deserialization into dbt objects based on the schema.

Who will this benefit?

Anyone consuming dbt json output, dbt Cloud, developers

[cmcarthur]

use a mix of dataclasses + https://github.com/s-knibbs/dataclasses-jsonschema to generate a JSON schema document for all of dbt's internal dbt objects

[cmcarthur]

should we generate & write out the json schema document to the target path?

[beckjake]

Fixed in #1589