Create new identity_inherit_role_assignment_v3 resource

[nikParasyr]

This creates new identity resource for inherited roles using the OS-INHERIT keystone API. Chose to create a new resource instead of adding this functionality to existing role assignment to:

• keep the resources clean and easy
• this is a seperate api, thus i prefer a new resource to manage it
• adding it to existing assignment would require updating the resource id and could impact users

Close #1372

[kayrus]

Thanks for the PR. The only drawbacks I see are duplicate code and documentation. The risk of human error is quite high, especially when someone makes a fix to a similar piece of code, as the duplicate part tends to be overlooked. In terms of code, this doesn't seem to be a major issue, as we can create additional functions that share the logic. However, when it comes to documentation, the only solution seems to be automatic documentation generation based on the source code. I believe we are not yet prepared to implement the autodocs, so I approve the PR.