Subnet resource and data source supports ipv6 id

[Lance52259]

What this PR does / why we need it:
The subnet id is missing in the subnet of ipv6.

Which issue this PR fixes:
(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)
reference: #989

Special notes for your reviewer:

Release note:

1. new ipv6_subnet_id parameter.

PR Checklist

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

Acceptance Steps Performed

make testacc TEST='./huaweicloud' TESTARGS='-run=TestAccVpcSubnetV1_ipv6'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run=TestAccVpcSubnetV1_ipv6 -timeout 360m -parallel 4
=== RUN   TestAccVpcSubnetV1_ipv6
=== PAUSE TestAccVpcSubnetV1_ipv6
=== CONT  TestAccVpcSubnetV1_ipv6
--- PASS: TestAccVpcSubnetV1_ipv6 (96.39s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       96.452s

[ShiChangkuo]

can we remove If the subnet is an IPv4 subnet, this parameter is not returned. ? I think it is unnecessary?

[Lance52259]

yeah, other IPv6 parameters also have this description.
None of them make sense.