Add support for Azure Workload Identity authentication

[cldmstr]

We run our vault instance in AKS and want to use the new Azure Workload Identity service to authenticate against Azure resources when using Azure keyvault for the unseal key handling.

This requires the Azure keyvault authentication to support the Workload Identity Federation mechanism.

Here is an implementation (leaning heavily on the Azure example) that I patched vault with and that runs successfully in our cluster.

Would it be possible to integrate this or something like it in the go-kms-wrapping module and then into the official vault release?

[cldmstr]

Pull request: #155

[cldmstr]

@jimlambrt As the PR has now been merged, should I close this as well? Is there any action on the vault side necessary to the new version integrated, or is that handled automatically?

[jimlambrt]

I just opened a PR in vault for this: hashicorp/vault#22994

[jimlambrt]

There's also a PR for boundary as well: hashicorp/boundary#3706