Implement workload identity-based token generation

[matt-carr]

Closes #22

Tested with the following test (not included because it wouldn't be particularly useful 🙃 )

  def test_acquire_access_token_mount_provided
    fed_token = "<snip>"
    file = Tempfile.new('azure_token')
    file.write(fed_token)
    file.close
    test_config = %[
      azure_use_workload_id true
      azure_federated_token_file_path #{file.path}
      azure_storage_account <snip>
      azure_oauth_tenant_id <snip>
      azure_oauth_app_id <snip>
      azure_container <snip>
      path log
      utc
      buffer_type memory
    ]
    d = create_driver(test_config)
    token = d.instance.instance_eval{acquire_access_token}
    assert token != nil
  end

Got a green run.

azurestorage_gen2: Token response: {"token_type"=>"Bearer", "expires_in"=>"86399", "ext_expires_in"=>"86399", "expires_on"=>"1724954202", "not_before"=>"1724867502", "resource"=>"https://storage.azure.com/", "access_token"=>"<snip>"}

Not ideal but at least it validates that we can get a token, which then feeds into the rest of the run.

[oleewere]

thanks for the contribution, sorry not reacted for the issue, just seeing it now