feat(RDS): add a data source to get the list of RDS SQLServer collations

[liwanting0517]

What this PR does / why we need it:

Which issue this PR fixes: (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged) fixes #xxx

Special notes for your reviewer:

Release note:

PR Checklist

• [x] Tests added/passed.
• [x] Documentation updated.
• [x] Schema updated.

Acceptance Steps Performed

=== RUN   TestAccDatasourceSQLServerCollations_basic
=== PAUSE TestAccDatasourceSQLServerCollations_basic
=== CONT  TestAccDatasourceSQLServerCollations_basic
--- PASS: TestAccDatasourceSQLServerCollations_basic (25.66s)
PASS

Process finished with the exit code 0