Add function for determining key_details

[ramonpetgrave64]

Summary

Client support for Rekor V2: sigstore-python #289

Partially addresses #1439

Inspired by https://github.com/sigstore/sigstore-python/pull/1432/files#diff-0287921ae266ddac61ace4f5d2ee5fa8e74495f6e3a7c231d3715a31679690b1R622-R651

Adds a function for determining the key_details of a certificate.

Note that this does not add direct support for signing with the various key types, but in the future will aid with sending requests to Rekor and verifying Entry body contents.

Release Note

• Added a function for determining the key_details of a certificate`.

Documentation

[jku]

The actual method looks pretty good (although maybe it could live in sigstore/_utils.py like other random methods like this).

Addresses #1439

This is not true since this only modifies the signing code (I assume the plan was to extend this to support verification as well later on) .

• I think we can re-use the code for signing and verification like it looks like you planned but it's worth noting that
  1. The signing code does not strictly speaking need this support (since there keytype is hard coded anyway and there's no plans to make that configurable at the moment)
  2. this re-use saves 7 lines of code
  3. the requirements for signing & verification might end up different: maybe we want to support the "not recommended" algorithms for verification but not for signing
• let's at least wait until #1432 so we can plug this into the verification code where it will be actually useful

[jku]

I'm going to approve this and handle #1439 in another PR using this

[jku]

/gcbrun