lazy load agate and add TYPE_CHECKING

[dwreeves]

resolves #125

This PR is related to the following open PR in dbt-core: #9744

TLDR: lazy-loading agate speeds up the load time of dbt by about 3.5%. Most instances of agate are unnecessary as dbt only uses it in a select few situations, and most instances of agate can be placed behind TYPE_CHECKING.

You can check that this works as intended by doing the following:

import sys
import dbt.adapters.base.impl
print([i for i in sys.modules if "agate" in i])

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX

[colin-rogers-dbt]

Thanks for raising this! Really interesting, we will probably want to cascade out to our implementing adapters as well

[dwreeves]

Yep, that will be the next step.

I'll get to your comment re: formatting changes, I think that must have been something with the pre-commit setup.

[dwreeves]

Fixed