refactor(nat): refactor the private SNAT rule resource code style

[ruwenqiang123]

What this PR does / why we need it:

Refactor the private SNAT rule resource 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/nat" TESTARGS="-run TestAccPrivateSnatRule_basic"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/nat -v -run TestAccPrivateSnatRule_basic -timeout 360m -parallel 4
=== RUN   TestAccPrivateSnatRule_basic
=== PAUSE TestAccPrivateSnatRule_basic
=== CONT  TestAccPrivateSnatRule_basic
--- PASS: TestAccPrivateSnatRule_basic (77.36s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/nat       77.407s

$ make testacc TEST="./huaweicloud/services/acceptance/nat" TESTARGS="-run TestAccPrivateSnatRule_cidr"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/nat -v -run TestAccPrivateSnatRule_cidr -timeout 360m -parallel 4
=== RUN   TestAccPrivateSnatRule_cidr
=== PAUSE TestAccPrivateSnatRule_cidr
=== CONT  TestAccPrivateSnatRule_cidr
--- PASS: TestAccPrivateSnatRule_cidr (77.05s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/nat       77.149s

• 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 <<<<<<