Add identity logging to ARM token exchange through IMDS

[Copilot]

This PR adds logging to show what identity is being used when performing token exchange through IMDS (Instance Metadata Service), making it easier to debug authentication issues.

Changes Made

Enhanced Token Exchange Logging

The ARMTokenForBinding function now logs the specific identity being used for authentication:

• Managed Identity with ClientID: Logs the specific client ID being used
• Managed Identity with ResourceID: Logs the specific resource ID being used
• System-assigned Managed Identity: Indicates when system-assigned identity is used
• Workload Identity: Logs the tenant ID and client ID being used

Example Log Output

INFO Attempting token exchange through IMDS using managed identity clientID="1b461305-28be-5271-beda-bd9fd2e24251"
INFO Attempting token exchange through IMDS using managed identity resourceID="/subscriptions/sub-name/resourceGroups/rg-name/providers/Microsoft.ManagedIdentity/userAssignedIdentities/1b461305-28be-5271-beda-bd9fd2e24251"
INFO Attempting token exchange through IMDS using system-assigned managed identity
INFO Attempting token exchange using workload identity tenantID="72f988bf-86f1-41af-91ab-2d7cd011db47" clientID="1b461305-28be-5271-beda-bd9fd2e24251"

Technical Implementation

• Modified the armTokenFetcher function type to include a logr.Logger parameter
• Updated ARMTokenForBinding function signature to accept and use the logger
• Enhanced CreatePullCredential function to pass logger context through the call chain
• Updated all related test functions to match the new signatures
• All existing tests continue to pass

This provides much-needed visibility into which identity is being used during token exchange operations, helping operators troubleshoot authentication issues more effectively.

Fixes #110.

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[matucker-msft]

I really have nothing to say. copilot did this well and plumbed the logger through to all the places and at least gave us the exact log line we wanted while debugging an issue today. This passes my sniff test, but I need 2 more approvals cause this is a copilot PR :D