Add a config option to suppress unmatched-source

[takeoverjp]

Thank you for the nice project!
This is my first PR for this project.
Therefore if I have any mistakes for contribution, please let me know...

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There are several reasons you might want this:

1. If the allow list represents the set sources for a project that have gone through some external approval process, such as vetting it with a legal department.
2. You're checking a single project in a workspace that shares its deny.toml for all members, but not all members have identical dependency sets.
3. You'd like to use deny.toml as part of a project template, and configure it with some default set of sources you find acceptable
4. ... others, for sure...

As it is, this warning isn't a huge deal, but is annoying/unhelpful if you don't care about it. I suspect that it's useful for catching typos or keeping configuration tight (and so I think "warn" is the right default for it), but there are enough reasons to want to turn it off that it seems justified to me for it to be an option.

It was easy to add support for a config property which controls the lint level for this check, so I just did that. I guess setting it to deny could be desirable in some cases, although it seems a little dodgy to me for various reasons... That said, I didn't see a reason to forbid that sort of thing, and allowing it to be configured as a LintLevel seemed more consistent.

Additional Information

• This PR is for Allow suppressing warning[unmatched-source]: allowed source was not encountered #781
• This PR modification is very similar with Add a config option to suppress "license was not encountered" warnings #368
• config name, unused-allowed-source is inspired by licenses.unused-allowed-licens in Add a config option to suppress "license was not encountered" warnings #368

[Jake-Shadle]

Thanks!

[takeoverjp]

@Jake-Shadle
Thank you so much for review and merge! 🎉
If decided, please let me know the release plan.

Does this project plan any periodic release?
Or should I wait for another important release?

[Jake-Shadle]

I'm doing a release today.

[takeoverjp]

Great!👍