feat(oms): add oms migration task and migration task group fields

[luoping-12345]

What this PR does / why we need it:
add oms migration task and migration task group fields

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:

add oms migration task and migration task group fields

PR Checklist

• Tests added/passed.

make testacc TEST="./huaweicloud/services/acceptance/oms" TESTARGS="-run TestAccMigrationTask_object"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/oms -v -run TestAccMigrationTask_object -timeout 360m -parallel 4
=== RUN   TestAccMigrationTask_object
=== PAUSE TestAccMigrationTask_object
=== CONT  TestAccMigrationTask_object
--- PASS: TestAccMigrationTask_object (212.35s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/oms       212.446s

make testacc TEST="./huaweicloud/services/acceptance/oms" TESTARGS="-run TestAccMigrationTaskGroup_prefix"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/oms -v -run TestAccMigrationTaskGroup_prefix -timeout 360m -parallel 4
=== RUN   TestAccMigrationTaskGroup_prefix
=== PAUSE TestAccMigrationTaskGroup_prefix
=== CONT  TestAccMigrationTaskGroup_prefix
--- PASS: TestAccMigrationTaskGroup_prefix (459.32s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/oms       459.397s

• Documentation updated.

• Schema updated.

• CheckDeleted.

  • a. During query operation (Read Context)
    aa. Resource not found
    >>>>>> Paste the screenshot here <<<<<<

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

[houpeng80]

/lgtm

[Jason-Zhang9309]

/lgtm
/approve

[github-ci-robot]

This PR has been approved by: @Jason-Zhang9309

/lgtm
/approve