Description

Adds resource and datasource ovh_cloud_project_gateway_interface.

Fixes #674 Replaces #358

Type of change

[ ] 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)
[x] Documentation update

[x] Test A: make testacc TESTARGS="-run TestAccCloudProjectGatewayInterfaceDataSource_basic"
[x] Test B: make testacc TESTARGS="-run TestAccCloudProjectGatewayInterface_basic"

[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 succesfully go mod vendor if I added or modify go.mod file