Add support for customizing the IMDS endpoint within ManagedIdentityCredential

[ahsonkhan]

Here’s my strawman proposal for the env var we’d add support for, which I am leaning towards: AZURE_IMDS_CUSTOM_AUTHORITY_HOST

Here are trade-offs with this name. Pro:

• Reflects more accurately what it is and does (self-describing).
• Builds on the learnings from other languages that shipped before (avoiding terminology that could be confusing).

Con:

• Inconsistent with other languages.

For context, the name used in other language SDKs is: AZURE_POD_IDENTITY_AUTHORITY_HOST