lazy load agate

[dwreeves]

resolves #1162

Problem

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.

Already implemented in dbt-adapters and dbt-core.

• lazy load agate and add TYPE_CHECKING dbt-adapters#126
• Make dbt CLI cold start load up to 8% faster with lazy loading of task modules and agate dbt-core#9744

Check performed (should return empty list):

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

Solution

from typing import TYPE_CHECKING

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

[VersusFacit]

Hey there @dwreeves ! I'm helping merge these this week. Awesome PRs. Could you fix the small merge conflicts on this? I'll get integration tests running too and then we can proceed to merge shortly!