Enable Nomad Server to mint & revoke Service Identity tokens

[shoenig]

As part of #6701, Nomad Server will need to be able to

• receive RPC requests from Nomad Agents for new SI tokens
• validate the request makes sense
  • does the requested policy match what the tasks in the alloc are configured for
• request Consul to create the SI token
  • plumb the created SI token through raft, making it available in the state store
• request Consul to revoke the SI token
  • plumb the revocation through raft, purging the token from the state store

[github-actions]

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