Add support for signing keys about nats-account-server HOT 1 CLOSED

This is about having the account-server sign JWTs this workflow will be handled externally by some user provided process.
Currently the server has an undocumented and usupported mechanism where if configured like this:

operatorjwtpath: "/srv/store/O/O.jwt",
systemaccountjwtpath: "/srv/store/O/accounts/SYS/SYS.jwt"
signrequestsubject: "sign.it"

http {
  port: 9090
  host: 0.0.0.0
},
store {
  dir: "/hostvolume/nats-account-server",
  readonly: false,
}
nats {
  servers: [nats://nats-server:4222]
  usercredentials: "/srv/creds/O/SYS/sys.creds"
}

Where the relevant part is the signrequestsubject, it will publish a message on that subject on the system account. Another process listening on the system account for that subject, will be presented with the JWT, and should reply with an operator signed JWT, which the server in turn will store. This effectively enables a workflow where some other process can perform verification of the request to sign the JWT, add limits.

from nats-account-server.