[Core] Add @runtime_checkable to TokenCredential protocol definitions

[laiapat]

Description

See #25175. With this PR, doing an isinstance check with an azure-identity credential and a TokenCredential protocol returns True -- that check errors today since these protocols aren't runtime checkable.

isinstance and issubclass checks aren't available without @runtime_checkable, but explicitly declaring the TokenCredential implementation (as shown below) could possibly satisfy static type checkers.

# from https://github.com/Azure/azure-sdk-for-python/blob/main/sdk/identity/azure-identity/azure/identity/_internal/get_token_mixin.py
...
from azure.core.credentials import TokenCredential
...
class GetTokenMixin(ABC, TokenCredential):

# from https://github.com/Azure/azure-sdk-for-python/blob/main/sdk/identity/azure-identity/azure/identity/aio/_internal/get_token_mixin.py
...
from azure.core.credentials_async import AsyncTokenCredential
...
class GetTokenMixin(abc.ABC, AsyncTokenCredential):

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[xiangyan99]

Let's split them into two separate PRs: core-only and non-core.

[azure-sdk]

API change check

APIView has identified API level changes in this PR and created following API reviews.

azure-core

[xiangyan99]

Do you want to add it into changelog?

[laiapat]

@xiangyan99 good point, it probably would be worth adding. I am just now noticing, though, that this isn't supported in Python 3.7. This'll need some reworking, unfortunately 🤔

[lmazuel]

Change my approve to comment, as we need 3.6 support. Maybe Krista has an idea.

[xiangyan99]

Change my approve to comment, as we need 3.6 support. Maybe Krista has an idea.

I guess what you wanted to say is we wanted 3.7 support?

[lmazuel]

@mccoyp we're dropping

Change my approve to comment, as we need 3.6 support. Maybe Krista has an idea.

I guess what you wanted to say is we wanted 3.7 support?

yeah :p

[laiapat]

Yeah, this won't work because 3.7 doesn't support it 😔 And it doesn't solve the motivating issue (see this comment), so we'll need a different approach completely.

I don't think this change (rather, a 3.7-compatible version) is a bad idea, because being able to do an isinstance check of a protocol implementation sounds desirable. But I can also close this PR for now -- @lmazuel what do you think?

[lmazuel]

@mccoyp , ok let's try to reproduce already, it should be possible. Once we understand that, we'll talk about solution.

[laiapat]

Identity CI failure is unrelated, and a fix is proposed here: #25787

Overriding check enforcer to merge since everything else is a-okay 🙂

[laiapat]

/check-enforcer override