[BUG] API consistency issue, ManagedIdentityCredential() does not read default values from environment variables, like other Credentials.

[InterpolationStation]

Library name and version

Azure.Identity 1.*

Describe the bug

The majority of the CredentialOptions classes obtain default values from Environment Variables, example client_id which allows applications to run on multiple platforms with runtime configuration injected via env vars (i.e. AZURE_CLIENT_ID, AZURE_TENANT_ID) without application code.

However, ManagedIdentityCredential doesn't follow this pattern.

For context, my org is the process of transitioning thousands of applications running across multiple hosting platforms to AKS. At the same time, we're moving from ManagedIdentity to WorkloadIdentity. All of our apps obtain credentials using DefaultAzureCredential(), as it allows env vars to control the runtime behavior without additional application code. Some hosting platforms assign multiple MSI's to compute nodes, requiring applications declare the client-id to use, others do not. Controlling this behavior via env vars avoids conditional application code.

Christopher recommended we not use DefaultAzureCredential() in production and replace it with `new ChainedTokenCredential(new ManagedIdentityCredential(), new WorkloadIdentityCredential()), however ManagedIdentityCredentialOptions doesn't obtain default values from env vars.

Expected behavior

ManagedIdentityCredential() parameterless ctor/ManagedIdentityCredentialOptions applies default values read from env vars, like other ...CredentialOptions classes. Example for client_id.

Actual behavior

ManagedIdentityCredential() parameter-less ctor applies null defaults, which is inconsistent with other credential options which apply default values from reading env vars.

Reproduction Steps

Define env var AZURE_CLIENT_ID=123
var cred = new ManagedIdentityCredential();
cred.ClientId is "123"

Environment

No response

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