feat: implement name server resource for domains

[PhilippeVienne]

Sure, here's the completed PR template with the provided diff details:

Description

This pull request implements a new feature: the name server resource for domains. Using the /nameServers/update endpoint, this change allows users to update the Name Servers (NS) of a domain registered at OVHCloud. This resource requires a new testing resource of a Domain where the NS can be changed. The changes have been tested on a separate account with a restricted token to ensure proper functionality.

The following files have been updated:

• README.md: Added OVH_DOMAIN_TEST environment variable for testing.
• provider.go: Registered the new ovh_domain_name_servers resource.
• provider_test.go: Included a check for OVH_DOMAIN_TEST.
• resource_domain_name_servers.go: Implementation of the new resource.
• resource_domain_name_servers_test.go: Test cases for the new resource.

Fixes #146 (issue)

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Improvement (improve existing resource(s) or datasource(s))
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation update

How Has This Been Tested?

The changes have been tested on a separate account with a restricted token. The following tests were conducted to verify the new functionality:

• [x] Test A: make testacc TESTARGS="-run TestAccDomainNameServers_Basic" – This test verifies the basic functionality of updating the Name Servers for a domain.

Test Configuration:

• Terraform version: terraform version: Terraform v1.8.3
• Existing HCL configuration you used:

resource "ovh_domain_name_servers" "foobar" {
	  domain = "terraform-test.ovh"
	  server {
	  	  host = "dns104.ovh.net"
	  }
	  server {
	  	  host = "ns104.ovh.net"
	  }
}

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings or issues
• [x] I have added acceptance tests that prove my fix is effective or that my feature works
• [x] New and existing acceptance tests pass locally with my changes
• [x] I ran successfully go mod vendor if I added or modify go.mod file