added saml configurations model

[pawel-macionczyk]

What does this PR do?

In the Terraform Provider for Datadog there is a missing feature for uploading SAML Metadata XML file.
The endpoint for doing that already exists in Datadog API: Upload IdP Metadata. It is also implemented in this repo.

The endpoint references to version V2 and has only one subpath. That's why I decided to create a new structure called "Saml Configurations" -> like the last but one subpath in endpoint. For now the structure contains only IdpMetadata but using this approach will let others extend this feature in the future.

The provided structure is created based on other models in api/datadogV2 directory and is used in the PR in Terraform Provider for Datadog repo.

Additional Notes

This PR should be merged first. Then I can reference a stable version of this repo in the go.mod in Terraform Provider for Datadog.

Review checklist

Please check relevant items below:

• This PR includes all newly recorded cassettes for any modified tests.

• This PR does not rely on API client schema changes.

  • The CI should be fully passing.

• Or, this PR relies on API schema changes and this is a Draft PR to include tests for that new functionality.

  • Note: CI shouldn't be run on this Draft PR, as its expected to fail without the corresponding schema changes.