Implement standalone feature

[Stupremee]

This PR introduces the new build feature standalone.
If this feature is enabled, cargo-deny doesn't use the cargo binary on the system, but the cargo crate.

Resolves #54

[Jake-Shadle]

Since this is the first feature for cargo-deny, you should probably modify https://github.com/EmbarkStudios/cargo-deny/blob/main/.github/workflows/ci.yaml#L103 to use the feature so that it gets coverage.

[repi]

nice!

For the published binaries of cargo-deny, should we also switch them to use this instead so they don't require Rust & Cargo to be installed? The extra build time for that CI publish step shouldn't be too bad. And then we can simplify the cargo-deny-action to reduce its size as it doesn't need the full Rust environment (tracked in EmbarkStudios/cargo-deny-action#3).

The nice thing about that approach is that users building cargo-deny locally (which obviously have Rust installed) will build and use with that, without having to build the Cargo crate, and same if one does cargo install cargo-deny to compile cargo-deny. But the standalone separate binaries are fully standalone

[Jake-Shadle]

Yah, that's probably a good idea, though I would need to add a new test to actually run it in an isolated environment, as there might be some dependencies with regards to environment variables being available that won't be set etc.

[Stupremee]

I think it's better if you add the changes to the release process, because I have no idea about the CI part

[Jake-Shadle]

Sorry for not merging this sooner, thanks a lot for this change!