Identity: 'AZURE_TOKEN_CREDENTIALS' env var support for specific credential names

[antkmsft]

This is a feature that was discussed within the Identity team, and certain details of the design may still change for all SDK languages (i.e. things like whether the possible values should be AzureCli vs AzureCliCredential), but I've got a bit of time and inspiration now to make this implementation maybe a bit ahead of time when design is finalized, but I am fine to make edits to this PR later, I think the main chunk of work is done - i.e. if we settle on AzureCliCredential or AZURE_CLI_CREDENTIAL instead of AzureCli value that is used in the initial iteration of this PR, it should be only a few lines of code to update.

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Release-wise, I am thinking of making a GA release from current main, and releasing this feature as Beta first - we could do 2 releases in July then - one GA, one Beta. I will check with the Identity team.

[Copilot]

Pull Request Overview

This PR introduces support for specifying a single credential type in DefaultAzureCredential via the AZURE_TOKEN_CREDENTIALS environment variable. Key changes include adding unit tests for various credentials, refactoring the credential selection logic in default_azure_credential.cpp, and updating the CHANGELOG.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
sdk/identity/azure-identity/test/ut/default_azure_credential_test.cpp	Adds parameterized tests for verifying log messages for specific credentials
sdk/identity/azure-identity/src/default_azure_credential.cpp	Refactors credential selection logic based on AZURE_TOKEN_CREDENTIALS
sdk/identity/azure-identity/CHANGELOG.md	Documents new supported AZURE_TOKEN_CREDENTIALS values