[Identity] Support Azure Arc user-assigned managed identity (UAMI) via MSAL

[g2vinay]

Background

MSAL for Java (msal4j) now supports acquiring tokens for user-assigned managed identities (UAMI) on Azure Arc (msal4j 1.26.0). Historically Azure Arc only supported system-assigned managed identity (SAMI). This issue tracks enabling Arc UAMI in azure-identity (ManagedIdentityCredential and DefaultAzureCredential).

Current behavior

• ManagedIdentityCredential routes Arc through ManagedIdentityMsalCredential → msal4j's ManagedIdentityApplication (only AKS federated-token is special-cased to AksExchangeTokenCredential). So the Arc token path already uses MSAL.
• However, ManagedIdentityCredential.getToken() contains an explicit upfront block: when a UAMI id is set and ManagedIdentityApplication.getManagedIdentitySource() is CLOUD_SHELL or AZURE_ARC, it throws CredentialUnavailableException ("User-assigned managed identity is not supported in AZURE_ARC..."). A separate block exists for SERVICE_FABRIC.
• A legacy ArcIdentityCredential also contains a UAMI guard (if (getClientId() != null) throw ...) that appears unused now that Arc is routed through ManagedIdentityMsalCredential.

Net effect: the token path is already MSAL-based; enabling Arc UAMI mainly requires removing the explicit AZURE_ARC guard plus a dependency uptake.

Scope of work

• Bump msal4j to >= 1.26.0.
• Remove AZURE_ARC from the UAMI guard in ManagedIdentityCredential.getToken() (keep the Cloud Shell and Service Fabric guards).
• Clean up the now-dead UAMI guard in ArcIdentityCredential (optional).
• Verify Arc UAMI token acquisition (client id, resource id, object id as supported).
• Update tests, class Javadoc, and CHANGELOG.

Key references

• sdk/identity/azure-identity/src/main/java/com/azure/identity/ManagedIdentityCredential.java
• sdk/identity/azure-identity/src/main/java/com/azure/identity/ArcIdentityCredential.java
• sdk/identity/azure-identity/src/main/java/com/azure/identity/ManagedIdentityMsalCredential.java