[Behavior Flags] Add Behavior Flag Framework to dbt-adapters

[mikealfare]

Housekeeping

• I am a maintainer of dbt-adapters

Short description

As an adapter maintainer, I want to use the Behavior Flag Framework in dbt-common, so that I can safely implement breaking changes while notifying users.

Acceptance criteria

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1. I can register behavior flags whenever an instance of BaseAdapter is created
2. Users can override the default setting of a behavior flag by setting the flag in the flags section of their dbt_project.yml
3. I can reference each flag as a property of the adapter, e.g. adapter.behavior.my_behavior_flag
4. Concrete adapters (e.g. dbt-postgres) can implement their own behavior flags; these flags will get incorporated into the same behavior flags that are implemented in dbt-adapters
5. Adapter maintainers have access to documentation that explains how to implement their own behavior flags
6. End user documentation is updated with this change; in particular, there's a new source of tracking, and a new motivation to allow tracking (a "vote" to not have legacy behavior removed if you're still using it)

Suggested testing

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1. Confirm that adding a behavior flag with no user override results in the expected default setting for the flag
2. Confirm that specifying a behavior flag in dbt_project.yml::flags results in overriding that flag

Security

This is effectively config, hence no more risky than current functionality.

Docs

We should make sure the behavior flag docs in docs.getdbt.com are updated. We also need to add docs for adapter maintainers.

Consequences

We should work closely with third party adapter maintainers to make sure they understand how to implement behavior flags in their own adapters as we will be asking them do so when appropriate.

[FishtownBuildBot]

Opened a new issue in dbt-labs/docs.getdbt.com: dbt-labs/docs.getdbt.com#6032