unify the nat gateway client to customize nat service endpoint by nat key

[ShiChangkuo]

What this PR does / why we need it:
unify the nat gateway client to NatGatewayClient, so we can customize nat service endpoint by nat key.

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

Special notes for your reviewer:

Release note:

PR Checklist

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

Acceptance Steps Performed

$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccNatSnatRule_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccNatSnatRule_basic -timeout 360m -parallel 4
=== RUN   TestAccNatSnatRule_basic
=== PAUSE TestAccNatSnatRule_basic
=== CONT  TestAccNatSnatRule_basic
--- PASS: TestAccNatSnatRule_basic (108.33s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       108.380s