[VPC] Add vpc_bandwidth_associate_v2 resource

[outcatcher]

Summary of the Pull Request

Add new resource opentelekomcloud_vpc_bandwidth_associate_v2

Fix minor problems with opentelekomcloud_vpc_bandwidth_v2 doc and RN

Part of #716 (still need a data source)

PR Checklist

• Refers to: FIP Bandwidth support #716
• Tests added/passed.
• Documentation updated.
• Schema updated.
• Release notes added.

Acceptance Steps Performed

=== RUN   TestBandwidthAssociateV2_basic
=== PAUSE TestBandwidthAssociateV2_basic
=== CONT  TestBandwidthAssociateV2_basic
--- PASS: TestBandwidthAssociateV2_basic (71.67s)
=== RUN   TestBandwidthAssociateV2_EIPv1
=== PAUSE TestBandwidthAssociateV2_EIPv1
=== CONT  TestBandwidthAssociateV2_EIPv1
--- PASS: TestBandwidthAssociateV2_EIPv1 (88.06s)
=== RUN   TestBandwidthAssociateV2_import
=== PAUSE TestBandwidthAssociateV2_import
=== CONT  TestBandwidthAssociateV2_import
--- PASS: TestBandwidthAssociateV2_import (46.76s)
PASS

Process finished with the exit code 0

[otc-zuul]

Build succeeded.

✔️ build-otc-releasenotes SUCCESS in 8m 51s
✔️ golangci-lint SUCCESS in 7m 58s
✔️ golang-make-test SUCCESS in 8m 57s
✔️ golang-make-vet SUCCESS in 8m 28s
✔️ tflint SUCCESS in 7m 44s
✔️ goreleaser-build SUCCESS in 7m 24s