Readd removed logic for deferral fail-over

[jtcohen6]

The logic added in #60 (to support dbt-labs/dbt-core#9199) was removed in #55.

I'm not sure if there's a good way to test this within dbt-adapters by itself...

Problem

Logic added to failover to defer_relation if this is not present in the target schema. (I realize that the logic I added in #60 wasn't quite right, either!)

Solution

Check to see if this exists (cache lookup), then fail over to using defer_relation instead for column type detection.

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

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.