Add Dependabot configuration for NuGet and git submodule

[JamieMagee]

What are you trying to accomplish?

Add Dependabot configurations for our own NuGet updater and our git submodule that pulls in NuGet.Client.

Anything you want to highlight for special attention from reviewers?

N/A

How will you know you've accomplished your goal?

Dependabot starts opening PRs

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.

[JamieMagee]

Can you do a dry-run to see if this works? :)

Will do!

[JamieMagee]

Can confirm with a dry run, all 3 configurations work. I expect quite a lot of NuGet package updates as well as a Git submodules update.