Add Ed25519 support to JWT

[pwolanin]

Comparing to other PHP JWT libraries this is an important missing feature, and fairly easy to implement

e.g. see: https://github.com/lcobucci/jwt/blob/4.2.x/src/Signer/Eddsa.php

This can be suggested for people on old versions of php or without libsodium (default in php 7.2+)
https://github.com/paragonie/sodium_compat

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Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[pwolanin]

@googlebot I signed it

[google-cla]

CLAs look good, thanks!

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[bshaffer]

@pwolanin this is great, thanks for adding this functionality!

Are you willing to write some tests? If not I'll try to find time to add some, but we definitely need tests before merging this PR.

[pwolanin]

@bshaffer can you approve the workflows so the tests run here?

Yes, I can add some tests, but I'd like to be sure they will pass here

[bshaffer]

@pwolanin I can approve to run the workflow once you've added tests. You haven't added any, so there's no reason to run them at this time...

[google-cla]

CLAs look good, thanks!

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CLAs look good, thanks!

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[pwolanin]

Pushed a first test and start on doc. I expect when this runs it will fail on older PHP versions that will need the shim installed

Accidentally approved

[bshaffer]

Fixed the test error in #344

[bshaffer]

Is there a reason (other than staying consistent with the other exceptions thrown from this method) that we are wrapping the libsodium exceptions in DomainException?

[pwolanin]

Just for consistency and so we are not adding new exception types for BC. Also the other exception types may not be defined in PHP without the library or shim present, so anyone trying to write generic try/catch code will have a harder time.