feat(vpn): VPN gateway resouce supports delete_eip_on_termination field

[profoundwu]

What this PR does / why we need it:
VPN gateway resouce supports delete_eip_on_termination field
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:

1. VPN gateway resouce supports delete_eip_on_termination field

PR Checklist

• Tests added/passed.

make testacc TEST="./huaweicloud/services/acceptance/vpn" TESTARGS="-run TestAccGateway_basic"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/vpn -v -run TestAccGateway_basic -timeout 360m -parallel 4
=== RUN   TestAccGateway_basic
=== PAUSE TestAccGateway_basic
=== CONT  TestAccGateway_basic
--- PASS: TestAccGateway_basic (459.73s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/vpn       459.785s


make testacc TEST="./huaweicloud/services/acceptance/vpn" TESTARGS="-run TestAccGateway_ deleteEipOnTermination"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/vpn -v -run TestAccGateway_deleteEipOnTermination -timeout 360m -parallel 4
=== RUN   TestAccGateway_deleteEipOnTermination
=== PAUSE TestAccGateway_deleteEipOnTermination
=== CONT  TestAccGateway_deleteEipOnTermination
--- PASS: TestAccGateway_deleteEipOnTermination (401.91s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/vpn       401.967s

• 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]

/approve

[github-ci-robot]

This PR has been approved by: @Jason-Zhang9309

/approve