Add indication for retrieving schema metadata for a list of relations and not all

[biswasbiplob]

Description

Use capability from adapters to indicate support for retrieving schema metadata for a list of relations, rather than always retrieving all the relations in a schema

This would enable dbt docs generate to only wait for schema metadata for the relations mentioned in the filters (--select, --exclude and --selector) and not waste time to try and fetch all.

Checklist

• You followed contributing section
• You kept your Pull Request small and focused on a single feature or bug fix.
• You added unit testing when necessary
• You added functional testing when necessary