signInstanceToken(payload: JWTPayload,key: InstanceSigningKey,claims: { issuer: string; audience: string; subject: string; },expiresIn: string): Promise<string>
Signs payload with the instance's key, from issuer (this instance) to
audience (the instance it is for), valid for expiresIn, with a random
jti. The identity tokens of POST /api/oauth/tokens and the events of
the outbox are signed with it.
key: InstanceSigningKey