add: fetch server config values via API

[coufalja]

Description

Cluster status API provides the set of server config values.

Motivation and Context

Useful for administration tools.
Example output:

{
  "id": "1",
  "version": "v0.3.2-43-gf927075-dirty",
  "tables": {
    "regatta-test": {
      "leader": "1",
      "raftTerm": "13"
    }
  },
  "config": {
    "api.address": "http://127.0.0.1:8443",
    "api.advertise-address": "http://127.0.0.1:8443",
    "api.cert-filename": "",
    "api.key-filename": "",
    "dev-mode": true,
    "log-level": "INFO",
    "maintenance.enabled": true,
    "maintenance.token": "**********",
    "memberlist.address": "0.0.0.0:7432",
    "memberlist.advertise-address": "",
    "memberlist.cluster-name": "default",
    "memberlist.members": [],
    "memberlist.node-name": "",
    "raft.address": "127.0.0.1:5012",
    "raft.compaction-overhead": "5000",
    "raft.election-rtt": 20,
    "raft.heartbeat-rtt": 1,
    "raft.initial-members": {
      "1": "127.0.0.1:5012"
    },
    "raft.listen-address": "",
    "raft.logdb": "tan",
    "raft.max-in-mem-log-size": "6291456",
    "raft.max-recv-queue-size": "0",
    "raft.max-send-queue-size": "0",
    "raft.node-host-dir": "/tmp/regatta/raft",
    "raft.node-id": "1",
    "raft.rtt": "50ms",
    "raft.snapshot-entries": "10000",
    "raft.snapshot-recovery-type": "",
    "raft.state-machine-dir": "/tmp/regatta/state-machine",
    "raft.wal-dir": "",
    "replication.address": "https://127.0.0.1:8444",
    "replication.ca-filename": "hack/replication/ca.crt",
    "replication.cert-filename": "hack/replication/server.crt",
    "replication.enabled": true,
    "replication.key-filename": "hack/replication/server.key",
    "replication.log-cache-size": 0,
    "replication.max-send-message-size-bytes": "4194304",
    "rest.address": "http://127.0.0.1:8079",
    "rest.read-timeout": "5s",
    "storage.block-cache-size": 16777216,
    "storage.table-cache-size": 1024,
    "tables.delete": [],
    "tables.names": [
      "regatta-test"
    ]
  }
}

How Has This Been Tested?