feat(evs): refactor evs snapshot resource with general code style

[deer-hang]

What this PR does / why we need it:

refactor evs snapshot resource with general code style

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:

PR Checklist

• Tests added/passed.

make testacc TEST='./huaweicloud/services/acceptance/evs' TESTARGS='-run TestAccEvsSnapshot_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/evs -v -run TestAccEvsSnapshot_basic -timeout 360m -parallel 4
=== RUN   TestAccEvsSnapshot_basic
=== PAUSE TestAccEvsSnapshot_basic
=== CONT  TestAccEvsSnapshot_basic
--- PASS: TestAccEvsSnapshot_basic (68.12s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/evs       68.174s

• 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
    

[github-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This PR has been approved by: @deer-hang

[jinyangyang222]

/lgtm