feat(cae): add new resource to manage certificate

[wuzhuanhong]

What this PR does / why we need it:
Add new resource (huaweicloud_cae_certificate) to manage certificate.

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:

1. add new resource.
2. add corresponding document and acceptance test.

PR Checklist

• Tests added/passed.

./scripts/coverage.sh -o cae -f TestAccCertificate_basic
Prepare to calculate the coverage the following command:
TF_ACC=1 go test "./huaweicloud/services/acceptance/cae" -v -coverprofile="./huaweicloud/services/acceptance/cae/cae_coverage.cov" -coverpkg="./huaweicloud/services/cae" -run TestAccCertificate_basic -timeout 360m -parallel 10
=== RUN   TestAccCertificate_basic
=== PAUSE TestAccCertificate_basic
=== CONT  TestAccCertificate_basic
--- PASS: TestAccCertificate_basic (48.05s)
PASS
coverage: 10.4% of statements in ./huaweicloud/services/cae
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/cae       48.127s coverage: 10.4% of statements in ./huaweicloud/services/cae

• Documentation updated.

• Schema updated.

• CheckDeleted.

  • a. During query operation (Read Context)
    aa. Resource not found
    

    ab. Related resources (parent resources) not found
    

  • b. During delete/disassociate/unbind operation (Delete Context)
    ba. Resource not found
    

    bb. Related resources (parent resources) not found
    

[Lance52259]

/approve
/lgtm

[github-ci-robot]

This PR has been approved by: @Lance52259

/approve
/lgtm