New apig group resource support

[Lance52259]

What this PR does / why we need it:
In order to manage APIs, environment variables, and custom responses, the APIG (API) group needs support.
The API group is the basic resource for other resources created or updated:

• API creation, publishing and offline
• Variable creation
• Custom response creation

TODO:

• API, environment variables, domain names and custom response list (for architecture) support.

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 api group resource support.
  a. support environment variables management.
2. new related document.
3. new related acc test.
  a. unit test for name and description.

PR Checklist

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

Acceptance Steps Performed

make testacc TEST='./huaweicloud/services/acceptance/apig' TESTARGS='-run=TestAccApigGroupV2_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/apig -v -run=TestAccApigGroupV2_basic -timeout 360m -parallel 4
=== RUN   TestAccApigGroupV2_basic
=== PAUSE TestAccApigGroupV2_basic
=== CONT  TestAccApigGroupV2_basic
--- PASS: TestAccApigGroupV2_basic (447.98s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/apig      448.041s

make testacc TEST='./huaweicloud/services/acceptance/apig' TESTARGS='-run=TestAccApigGroupV2_variables'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/apig -v -run=TestAccApigGroupV2_variables -timeout 360m -parallel 4
=== RUN   TestAccApigGroupV2_variables
=== PAUSE TestAccApigGroupV2_variables
=== CONT  TestAccApigGroupV2_variables
--- PASS: TestAccApigGroupV2_variables (448.65s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/apig      448.787s