Add Ed25519 OIDs and signature algorithm mappings

[catbref]

This allows easier use of JcaContentSignerBuilder to build X509 certificates like so:

JcaX509v3CertificateBuilder certBuilder = new JcaX509v3CertificateBuilder(issuer, serial, notBefore, notAfter, subject, publicKey);
ContentSigner contentSigner = new JcaContentSignerBuilder("SHA512withEd25519").setProvider(BC).build(privateKey);
X509Certificate cert = new JcaX509CertificateConverter().setProvider(BC).getCertificate(certBuilder.build(contentSigner));

OIDs from https://tools.ietf.org/html/draft-ietf-curdle-pkix-07

[Valodim]

Hey there. Are you sure you wanted to create this PR against the OpenKeychain fork of bouncy-castle, and not the upstream repo at https://github.com/bcgit/bc-java? :)

[catbref]

Hi!
I wasn't sure which git to PR against.
I don't know if it's only of use to you because BC aren't/won't merge your own PR?
There doesn't seem to be any action at BC regarding this and I think if you merge my PR then it updates your PR to BC anyway?
Maybe you have a better idea than me!
Also thanks for doing the major work on this!

[catbref]

Hello again...

Did you ever submit a PR to bouncy-castle for your EDDSA commits?
I can't seem to find one at their repository.
If not, do you mind I create a PR against their master?
Your username would still be mentioned in the PR if that's a concern?