Implement retry logic for IMDS managed identity

[chlowell]

Closes #2729 by aligning the retry policy configuration for our IMDS client with the published guidance. Our timing is close but doesn't exactly match the recommended curve:

attempt	occurs at (s)	recommended
1	0	0
2	2	2
3	8	8
4	19	22
5	40	52
6	70	-

[Copilot]

Pull Request Overview

This PR implements retry guidance for ManagedIdentityCredential to align with Microsoft's IMDS (Instance Metadata Service) recommendations. The changes ensure proper retry behavior for Azure VM managed identity authentication.

• Configures custom retry status codes for IMDS endpoints (404, 410, 429, 500-511)
• Sets exponential retry options following IMDS guidance (initial delay: 1340ms, max retries: 6, max elapsed: 72s)
• Passes pipeline options through the credential chain for VM credentials

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
sdk/identity/azure_identity/src/virtual_machine_managed_identity_credential.rs	Implements IMDS retry guidance with custom status codes and exponential retry configuration
sdk/identity/azure_identity/src/imds_managed_identity_credential.rs	Adds pipeline_options parameter to support custom retry configuration
sdk/identity/azure_identity/src/app_service_managed_identity_credential.rs	Passes None for pipeline_options to maintain default retry behavior
sdk/identity/azure_identity/CHANGELOG.md	Documents the bug fix for IMDS retry guidance