PKCS12: return 'friendly name' with PKCS12KeyAndCertificates API

[felixfontein]

This PR provides a new API asked for in #5760 to return the 'friendly name' from a PKCS12 file, which happens to be the name of the "main" certificate with private key that was provided to the serialize_key_and_certificates API when creating a PKCS12. This API brings cryptography's API more to par with the API provided by PyOpenSSL. (Now the only remaining differences I'm aware of are related to the encryption algorithm / configuration.)

As suggested by @reaperhulk in #5760 (comment), adds a new API which also returns the 'friendly name'. I've adjusted the name slightly to load_key_and_certificates_with_name since the storing API (serialize_key_and_certificates) also uses name. I've put name as the first return value of the new API since that's the order of parameters serialize_key_and_certificates uses. (If these two things should change, I'm happy to do that :) )

The old API load_key_and_certificates now basically calls the new API and ignores the name.

Fixes #5760.

[reaperhulk]

This needs docs, changelog, and tests that exercise the new codepath (codecov not reporting is a common problem, but if it did report this would fail coverage).

[reaperhulk]

Two big comments:

1. Let's not make a new serialization API for now. Let's just add a test vector that has multiple aliases and be able to parse that.
2. I think we need to return a PKCS12 object now, this multiply-nested tuple API is too much.

A true PKCS12 object is ridiculous (see: https://www.bouncycastle.org/docs/pkixdocs1.5on/org/bouncycastle/pkcs/PKCS12PfxPduBuilder.html) so maybe what we want is more along the lines of

class PKCS12KeyAndCertificates:
    @getter
    def key -> PRIVATE_KEY_TYPES: ...

    @getter
    def cert -> PKCS12Certificate: ....

    @getter
    def additional_certs -> List[PKCS12Certificate]: ...

class PKCS12Certificate:
    @getter
    def friendly_name -> bytes: ...

    @getter
    def certificate -> x509.Certificate: ...

[felixfontein]

@reaperhulk I guess key and cert in PKCS12KeyAndCertificates should be typing.Optional[]?

[reaperhulk]

Yeah they're not required so they should be optionals. That's what I get from writing an interface in a GitHub comment 😄

[reaperhulk]

All the new test vectors should be documented in test-vectors.rst describing what they contain. We have a section named Custom PKCS12 Test Vectors you can add them to.

[reaperhulk]

Ready for rebase

[felixfontein]

I don't think the cancelled test is related to this PR.

stale

[felixfontein]

@reaperhulk thanks a lot for guiding this new feature, and for reviewing and merging!