New APIG custom authorizer resource supported

[Lance52259]

What this PR does / why we need it:
A mechanism defined with custom rules for API Gateway to verify the validity and integrity of requests initiated by API callers. The mechanism is also used for backend services to verify the requests forwarded by API Gateway.
The following two types of custom authentication are provided:

• Frontend custom authentication: A custom authorizer is configured with a function to authenticate requests for an API.
• Backend custom authentication: A custom authorizer can be configured to authenticate requests for different backend services, eliminating the need to customize APIs for different authentication systems and simplifying API development. You only need to create a function-based custom authorizer in API Gateway to connect to the backend authentication system.

Which issue this PR fixes:
(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)
reference: #1249

Special notes for your reviewer:

Release note:

1. New custom authorizer resource, which support fornt and backend auth management.
2. Related acc unit test supported.
3. New document supported.

PR Checklist

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

Acceptance Steps Performed

make testacc TEST='./huaweicloud/services/acceptance/apig' TESTARGS='-run=TestAccApigCustomAuthorizerV2_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/apig -v -run=TestAccApigCustomAuthorizerV2_basic -timeout 360m -parallel 4
=== RUN   TestAccApigCustomAuthorizerV2_basic
=== PAUSE TestAccApigCustomAuthorizerV2_basic
=== CONT  TestAccApigCustomAuthorizerV2_basic
--- PASS: TestAccApigCustomAuthorizerV2_basic (475.90s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/apig      475.968s

[Lance52259]

Pending sdk supports custom authorizaiton.

[chengxiangdong]

Suggest to use ^[A-Za-z][\\w]{2,63}$ for more conciseness.

[Lance52259]

thx, the \w contain 0-9.