feat(workspace/app): add new resource to manage image server

[wuzhuanhong]

What this PR does / why we need it:

Add new resource to manage image server of the Workspace APP.

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 related document and acceptance test.

PR Checklist

• Tests added/passed.

 ./scripts/coverage.sh -o workspace -f TestAccResourceAppImageServer_basic
Prepare to calculate the coverage the following command:
TF_ACC=1 go test "./huaweicloud/services/acceptance/workspace" -v -coverprofile="./huaweicloud/services/acceptance/workspace/workspace_coverage.cov" -coverpkg="./huaweicloud/services/workspace" -run TestAccResourceAppImageServer_basic -timeout 360m -parallel 10
=== RUN   TestAccResourceAppImageServer_basic
=== PAUSE TestAccResourceAppImageServer_basic
=== CONT  TestAccResourceAppImageServer_basic
--- PASS: TestAccResourceAppImageServer_basic (814.76s)
PASS
coverage: 8.4% of statements in ./huaweicloud/services/workspace
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/workspace 814.848s        coverage: 8.4% of statements in ./huaweicloud/services/workspace

• Documentation updated.

• Schema updated.

• CheckDeleted.

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

  • b. During delete/disassociate/unbind operation (Delete Context)
    ba. Resource not found
    >>>>>> Paste the screenshot here <<<<<<