Support for configuring SSO on Terraform Cloud

[tmatilai]

Use-cases

As automation and IaC is the new black, we would like to manage all Terraform Cloud configurations with Terraform.
Especially the SSO/SAML related resources and data sources are missing now.

So this issue is basically the same as #819 but for Terraform Cloud instead of Terraform Enterprise.

Attempted Solutions

Now the SSO configuration on TFC has to be done manually after configuring the IdP.

Proposal

Add needed resources and data sources for managing and fetching the SSO configurations. Maybe something like tfe_organizations_saml_configurations, as the API endpoint to fetch the configs seems to be https://app.terraform.io/api/v2/organizations/<ORG_NAME>/saml-configurations.

I'm happy to try to help implementing this, but as the API endpoints seem to be undocumented, and this of course requires the SDK to support the functionality, help is needed from the dev team, too.