Ability to express user-assigned managed identity in IConfiguration

[mattchenderson]

Library or service name.
Microsoft.Extensions.Azure

Is your feature request related to a problem? Please describe.
When using a configuration source to drive a connection, I would like to be able to reference a user-assigned identity. The model today accounts for system-assigned only. That's the default case, and a client ID option exists in conjunction with a secret to form a client credential. Just adding a solo client ID could create some confusion there, so it needs to be explicit. That's especially true considering deployment environments.

Proposal (CC @pakrym) : Add an explicit credential property (possibly useable in other areas), which can be set to "managedidentity" and indicates the ManagedIdentityCredential type should be used. A clientId property is optional and if set indicates the used-assigned identity as per the ManagedIdentityCredential constructor. In absence of clientId, the system-assigned identity should be used. In practice, setting credential on its own is unlikely since omitting it entirely would provide the same result + local options per DefaultAzureCredential underneath.

Using a Blob as an example (json config):

{
  "MyStorageConnection": {
      "serviceUri": "…",
      "credential": "managedidentity",
      "clientId": "…"
  }
}

[jsquire]

Thank you for your feedback. Tagging and routing to the team member best able to assist.

[pakrym]

Done in #18459.