feat(iam): add new resource identity policy

[Jason-Zhang9309]

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.

go test ./huaweicloud/services/acceptance/iam -v -run TestAccIdentityPolicy_basic -timeout 360m -parallel 4
=== RUN   TestAccIdentityPolicy_basic
=== PAUSE TestAccIdentityPolicy_basic
=== CONT  TestAccIdentityPolicy_basic
--- PASS: TestAccIdentityPolicy_basic (32.39s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/iam       32.548s

• Documentation updated.

• Schema updated.

• CheckDeleted.

  • a. During query operation (Read Context)
    aa. Resource not found
    it's checked, but can't upload the screenshot here

  • b. During delete/disassociate/unbind operation (Delete Context)
    ba. Resource not found
    it's checked, but can't upload the screenshot here

[github-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This PR has been approved by: @Jason-Zhang9309

[houpeng80]

/lgtm