Add support for x5t and x5t#S256 header

[hieuk09]

Description

Close #562

Checklist

Before the PR can be merged be sure the following are checked:

• There are tests for the fix or feature added/changed
• A description of the changes and a reference to the PR has been added to CHANGELOG.md. More details in the CONTRIBUTING.md

[anakinj]

@hieuk09 Did you have some reference for how this should work? Could take a look at that to educate myself a bit.

[anakinj]

@hieuk09 Did you have some reference for how this should work? Could take a look at that to educate myself a bit.

Now when looking closer at the changes I found some links...

[hieuk09]

Thank you for the detailed review. I'll take a look and fix all the comments tomorrow.

[hieuk09]

@anakinj could you have another review?

[anakinj]

I think the JWK approach is interesting. Im a bit concerned that we are cutting some corners here, as the x5* fields are pretty certificate specific and we are currently only dealing with keypairs.

[anakinj]

@hieuk09 Was there something pending for this one still. To my understanding the functionality for picking the key field is totally optional now?

[hieuk09]

@anakinj I think there are no pending tasks for this PR, the key field should be totally optional now.