Add azuread_invitation resource

[romainDavaze]

Implementing azuread_invitation resource to invite guest users.

Acceptance tests may not pass because of a timing issue between the creation of the guest user and its deletion.

This is issue is addressed here. Ideally, we need to wait for this PR to be merged in order to make these tests pass.

[manicminer]

Rebased to resolve conflicts, updated resource and docs to dry it out a bit and add the user_type property, along with some more tests.

Thanks @romainDavaze for this implementation!

[katbyte]

as discussed internally LGTM 🙌

[manicminer]

Thanks @katbyte! 🍕

Test results:

[github-actions]

This functionality has been released in v2.1.0 of the Terraform Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

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