[Feat.] APIGW: Add new resource opentelekomcloud_apigw_certificate_v2

[artem-lifshits]

Summary of the Pull Request

Add new APIGW resource:

• opentelekomcloud_apigw_certificate_v2

PR Checklist

• Refers to: [APIG] Request for New Features: Enhancements for API Gateway (APIG) Management  #2730
• Tests added/passed.
• Documentation updated.
• Schema updated.
• Release notes added.

Acceptance Steps Performed

=== RUN   TestAccCertificate_instance
=== PAUSE TestAccCertificate_instance
=== CONT  TestAccCertificate_instance
--- PASS: TestAccCertificate_instance (507.95s)
PASS

Process finished with the exit code 0


=== RUN   TestAccCertificate_basic
=== PAUSE TestAccCertificate_basic
=== CONT  TestAccCertificate_basic
--- PASS: TestAccCertificate_basic (50.13s)
PASS

Process finished with the exit code 0

=== RUN   TestAccCertificate_instanceWithRootCA
=== PAUSE TestAccCertificate_instanceWithRootCA
=== CONT  TestAccCertificate_instanceWithRootCA
--- PASS: TestAccCertificate_instanceWithRootCA (508.33s)
PASS

Process finished with the exit code 0

[otc-zuul]

Build succeeded.
https://zuul.otc-service.com/t/eco/buildset/56e2a3032daa4e0a90335bcf5a2fa7a1

✔️ build-otc-releasenotes SUCCESS in 4m 02s
✔️ noop SUCCESS in 0s
✔️ otc-golangci-lint SUCCESS in 3m 09s
✔️ golang-make-test SUCCESS in 4m 03s
✔️ golang-make-vet SUCCESS in 2m 41s
✔️ tflint SUCCESS in 1m 46s
✔️ goreleaser-build SUCCESS in 7m 21s