feat: add cargo deny

[CrabNejonas]

This PR adds cargo-deny as a CI check to ensure our dependency tree only contains MIT/Apache-2.0 compatible licenses.

I also ran cargo-deny as part of setting this up ofc and it seems good except for 3 questions we have to answer:

• is BSD-3-Clause compatible with MIT/Apache-2.0?
• is OpenSSL license compatible with MIT/Apache-2.0?
• is Zlib license compatible with MIT/Apache-2.0?

[beanow-at-crabnebula]

I think OpenSSL/SSLeay, BSD and Zlib are fine to use as dependencies.

But zlib does put some limitations on modifying, so if we need to, I think we'll have to do a bunch of forking work to keep that contained.

Similarly the old OpenSSL/SSLeay license requires some notice, but can be used as dependency.

[CrabNejonas]

But zlib does put some limitations on modifying, so if we need to, I think we'll have to do a bunch of forking work to keep that contained.

Zlib is only used in adler32 which I don't expect us to modify like ever, so that's fine

Similarly the old OpenSSL/SSLeay license requires some notice, but can be used as dependency.

Something we have to put into the readme or similar?