Initial `dotnet_sdk` updater

[JamieMagee]

What are you trying to accomplish?

Add support for .NET SDK updates in global.json. Closes #2442.

Anything you want to highlight for special attention from reviewers?

Related PRs:

• https://github.com/dependabot/cli/pull/367
• https://github.com/dependabot/smoke-tests/pull/234

How will you know you've accomplished your goal?

Checklist

• [x] I have run the complete test suite to ensure all tests and linters pass.
• [x] I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• [x] I have written clear and descriptive commit messages.
• [x] I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• [x] I have ensured that the code is well-documented and easy to understand.