Introduce a separate field for the signature key of player certificates

[erickskrauch]

Hello.

We're adopting player certificates functionality into Ely.by's authlib-injector integration. But here is a problem: we would like to have separate keys for properties and players' certificates signing. Right now it's impossible and we have to use the same keys. It would be nice to have an additional field in the metadata response to specify the player's certificates signing key.

It would be perfect to introduce new fields propertiesPublicKeys and playersCertificatesPublicKeys as arrays to allow server owners to rotate keys by exposing a few keys during the migration period.

[erickskrauch]

@evan-goode has already done the job to support multiple keys on #229.