[CT-1621] [Feature] de-globalize things through contextvar

[ChenyuLInx]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

Describe the feature

We have a few things inside dbt-core that are python global objects. Some examples are active_user, EVENT_MANAGER. With the future of dbt-server running multiple dbt-core invocations at the same time, we should make sure those multiple invocation don't overwrite those objects.

In the example here, contextvars is a great way of dealing with global variables during concurrency.

We should make sure we move the usage of all global object in dbt-core into contextvars

[jtcohen6]

Another funny one to add to the list of unnecessarily global objects: selected_resources. We already have an open issue for that here: #6009. Thoughts on keeping it as a separate issue/ticket, versus pulling it into this one?

And then, of course, there's the one singleton to rule them all: adapters (#5533 (comment)). I don't know if we'd want to take a contextvars-based approach there, rather than really reconsidering / refactoring how adapters are registered + initialized.

[MichelleArk]

Another global flag to be aware of is USE_COLORS, which is initialized as part of GLOBAL_FLAGS Namespace in flags.py, and used in ui.color