Ensure license headers adher to Cargo manifest

[rzadp]

• Allows us to ensure that if a license header exists, it matches the Cargo manifest license.
• It's needed by Compare license to Cargo.toml #44
• Introduce an option to filter by file extensions.
  • So far, for the purpose of ensuring licenses, the tool has been used with a shell glob. However, we need to use an already-existing functionality of traversing the directories, so that we can propagate the Cargo manifest license down the line.
  • With this, we can close Revamp the include parameter #45.
• Update the rust-crate-scanner to properly parse inherited properties in Cargo.toml.
  • It does NOT completely finish the matter of rust-crate-scanner does not handle inherited properties #56, but at least it doesn't crash.

[rzadp]

@paritytech/opstooling A ping for review :)

[mutantcornholio]

Feels like a realy weird type, and I can't find how it's used.

[mutantcornholio]

Okay, so now we return Inheritable<> from the scanner, which is {workspace: true}, but do we somehow use it?
If we wanted just to avoid crash, why not return null there?
If we wanted to resolve inherited properties, why didn't we do it in rust scanner, why expose it to ts part?

[rzadp]

Great points.

I didn't want to resolve inherited properties, I created an issue and didn't want to spend time on it, because this functionality is not being used for years.

But now that I think about it again, it should be rather simply to resolve it now.
I'll take another look at it. If not, I'll return null as you suggest.

[rzadp]

@mutantcornholio I redid it, now I just return null instead of those inheritable types without using them.

Added a new test target - a crate with inherited properties.

[mutantcornholio]

LGTM