[CT-3118] Design Core Changes for Metadata-Driven Freshness

[peterallenwebb]

We would like to unblock work on the adapter changes necessary for supporting metadata-driven freshness checks. This will allow us to move forward more rapidly and in parallel with core and adapter implementations. To do so, we need to review the existing interface between core and adapters, as it relates to source freshness checks, and then specify a concrete set of interface changes that will allow core and adapters to work together to determine source freshness via metadata when a supporting mechanism is available in the DWH.

Acceptance Criteria

• A written design document, describing in detail any adapter changes which will be necessary to support source freshness via metadata mechanisms.
• The design should include complete details of any new macros that are needed, or changes to the adapter Python class implementations which are needed to support metadata-driven freshness.
• The design should be specific and detailed enough to provide, at least in theory, all the information needed for adapter maintainers and core engineers to work together in parallel to complete changes, without further engineering design of the interface.

[peterallenwebb]

There are only two adapter-related changes that will be necessary to support this work.

1. We will add a new static boolean property to the base adapter called METADATA_FRESHNESS_SUPPORT. It will default to False.
2. When an adapter implementation overrides this property and sets it to True, and the user has requested that freshness for a source be determined via metadata, dbt-core will look for a macro called collect_freshness_via_metadata(sources). The new macro will accept a List[BaseRelation] and return source, timestamp for the most recent modification of each relation.

Implementation will be done in #7012.