feat(MRS): add a new datasource to get flavors

[hmbig2]

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

Running tool: /usr/local/go/bin/go test -timeout 9000000000s -run ^TestAccDatasourceMrsFlavors_basic$ github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/mrs -v

=== RUN   TestAccDatasourceMrsFlavors_basic
=== PAUSE TestAccDatasourceMrsFlavors_basic
=== CONT  TestAccDatasourceMrsFlavors_basic
--- PASS: TestAccDatasourceMrsFlavors_basic (38.24s)
PASS
ok  	github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/mrs	38.293s