feat(RDS): add rds pg sql limits data source

[houpeng80]

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

• Tests added/passed.
• Documentation updated.
• Schema updated.

Acceptance Steps Performed

make testacc TEST=./huaweicloud/services/acceptance/rds/ TESTARGS='-run TestAccDataSourceRdsPgSqlLimits_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/rds/ -v -run TestAccDataSourceRdsPgSqlLimits_basic -timeout 360m -parallel 4
=== RUN   TestAccDataSourceRdsPgSqlLimits_basic
=== PAUSE TestAccDataSourceRdsPgSqlLimits_basic
=== CONT  TestAccDataSourceRdsPgSqlLimits_basic
--- PASS: TestAccDataSourceRdsPgSqlLimits_basic (813.47s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/rds       813.516s

[github-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This PR has been approved by: @houpeng80

[Jason-Zhang9309]

/lgtm