Rename xrole and app arguments for fgs

[niuzhenguo]

What this PR does / why we need it:
It's a bit confused that app means package in terrafrom and agency means xrole

Which issue this PR fixes:
(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)
fixes #1076

Special notes for your reviewer:

Release note:

PR Checklist

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

Acceptance Steps Performed

TF_ACC=1 go test ./huaweicloud/ -v -run=TestAccFgsV2Function_basic -timeout 360m -parallel 4
=== RUN   TestAccFgsV2Function_basic
=== PAUSE TestAccFgsV2Function_basic
=== CONT  TestAccFgsV2Function_basic
--- PASS: TestAccFgsV2Function_basic (56.48s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       56.499s