feat: add Jwk::from_decoding_key

[eyakubovich]

Allow for constructing a Jwk from a decoding key.
This allows it to be created from a DER encoded file, for example.

This patch refactors some Jwk internals to reduce code duplication.

[Keats]

That will need to wait a bit, we need to merge #452 first that has a lot of change that would overlap with some of your changes.

[eyakubovich]

@Keats @arckoor PTAL. I removed JwkUtils (technically those functions are not JWK specific) and put everything up at the same level. However I added separate structs from RSA and EC component extraction, each with a from_private and from_public functions.

As an aside, why are function pointers used instead of traits?

[arckoor]

As an aside, why are function pointers used instead of traits?

It was what happened to work when I implemented it, I didn't explicitly choose not to use traits, there wasn't any deep logic behind it.
I didn't necessarily plan for more stuff being added in the future

[eyakubovich]

Ping. I addressed the comments. Please unlock the workflows to run the checks.

[arckoor]

Please rebase to latest master. A bunch of other PRs are also adding more utility functions to JwkUtils, so I think keeping that struct around is better in the long run, the CryptoProvider grows uncomfortably big otherwise. Though renaming it to something like KeyUtils or similar wouldn't hurt