enable crossdb column metadata with ra3 nodes

[dataders]

resolves #736

Problem

R3-node users cannot fetch column metadata for tables & views outside of the current database.

Solution

Similar to @jiezhen-chen's approach in #428: use svv_all_columns instead of information_schema."columns".

While we're here, I have a hunch that @aiguofer's dbt-labs/dbt-core#2754 and it's usage of pg_catalog.svv_external_columns can be dropped. Still to be verified.

Questions

1. Does this fix things for RA3 node cross-db scenarios?
2. Does it break things for non-RA3 scenarios?
3. What is the performance impact of such a change?
4. Is it correct to assume that svv_all_columns includes the results of svv_external_columns?
5. To ameliorate 2 and 3, should this svv_all_columns only be used if the relation-to-be-fetched's database does not match the one to which dbt is currently connected? (similar: [CT-1122] [CT-1116] [Bug] adapter.get_relation for relations in other databases [RA3] #179 (comment))

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

[VolkerSchiewe]

4. Is it correct to assume that svv_all_columns includes the results of svv_external_columns?

I can confirm that svv_all_columns contains column information of external schemas as well.