[Feature] Subnet supports enable IPv6

[Lance52259]

What this PR does / why we need it:

1. API supports enable IPv6 of VPC subnet.
2. New parameter updation in datasource but code of acc test doesn't meet the design specifications.

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

NONE

Special notes for your reviewer:

Release note:

1. new 'ipv6_enable', 'ipv6_cidr' and 'ipv6_gateway' parameters.
2. refactor datasource test.
3. new 'ipv6_enable', 'ipv6_cidr' and 'ipv6_gateway' parameters description in documents.

PR Checklist

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

Acceptance Steps Performed

Resource: check IPv6 enable

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 (90.44s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       90.514s

Data Source: basic check

make testacc TEST='./huaweicloud' TESTARGS='-run=TestAccVpcSubnetV1DataSource_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run=TestAccVpcSubnetV1DataSource_basic -timeout 360m -parallel 4
=== RUN   TestAccVpcSubnetV1DataSource_basic
=== PAUSE TestAccVpcSubnetV1DataSource_basic
=== CONT  TestAccVpcSubnetV1DataSource_basic
--- PASS: TestAccVpcSubnetV1DataSource_basic (65.90s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       65.950s

Data Source: check IPv6 enable

make testacc TEST='./huaweicloud' TESTARGS='-run=TestAccVpcSubnetV1DataSource_ipv6'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run=TestAccVpcSubnetV1DataSource_ipv6 -timeout 360m -parallel 4
=== RUN   TestAccVpcSubnetV1DataSource_ipv6
=== PAUSE TestAccVpcSubnetV1DataSource_ipv6
=== CONT  TestAccVpcSubnetV1DataSource_ipv6
--- PASS: TestAccVpcSubnetV1DataSource_ipv6 (72.46s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       72.513s

[ShiChangkuo]

how to understand connection here?

[Lance52259]

This description is confusing to read. Updation has completed.

[ShiChangkuo]

does the file is auto-generated by go get xxx?

[Lance52259]

yes