feat(database): use service discovery for primary address

Related to #890 (closed)

What does this MR do?

It introduces a dns package that can resolve SRV and A types of records from a DNS server. It also adds a new discovery section to the database configuration to enable service discovery for a primary database host.

How to test locally

The setup requires a DNS server that can reply to SRV and A requests. The easiest way is to run a PostgreSQL container and install consul:

1. Create a volume docker volume create postgres_data
2. docker run --name postgres-registry -d --restart=always -p 55432:5432 -e POSTGRES_USER=registry -e POSTGRES_PASSWORD=apassword -e POSTGRES_DB=registry_dev -e PGDATA=/var/lib/postgresql/pgdata -v postgres_data:/var/lib/postgresql/pgdata postgres:12-alpine
3. ensure the database registry_dev exists, e.g. using psql : PGPASSWORD=apassword psql -h 127.0.0.1 -U registry -p 55432 -d registry_dev -c 'select 1';
4. Create a consul directory and add some configuration settings:
5. mkdir -p consul/data
6. cd consul
7. touch consul.json
8. Add the following to consul.json

{
  "data_dir": "consul/data",
  "enable_local_script_checks": true,
  "services": [
    {
      "id": "primary-55432",
      "name": "registry-primary",
      "port": 55432,
      "address": "127.0.0.1",
      "tags": [
        "primary"
      ],
      "checks": [
        {
          "args": ["psql", "-U", "registry", "-h", "127.0.0.1", "-p", "55432", "-d", "registry_dev", "-c", "select 1"],
          "interval": "2s"
        }
      ]
    }
  ]
}

1. Consul will run in the terminal and the logs can be seen when a query is attempted:

dig +short @127.0.0.1 -p 8600 primary.registry-primary.service.consul -t SRV
1 1 55432 7f000001.addr.dc1.consul.

2023-04-05T17:54:09.221+1000 [DEBUG] agent.dns: request served from client: name=primary.registry-primary.service.consul. type=SRV class=IN latency="260.625µs" client=127.0.0.1:59990 client_network=udp
2023-04-05T17:54:10.818+1000 [DEBUG] agent: Check status updated: check=service:primary-55432 status=passing
2023-04-05T17:54:12

---

Now add the following to your registry config.yml file under the database section

database:
  enabled: true
  password: "apassword"
  dbname:   "registry_dev"
  sslmode:  "disable"
  discovery:
    enabled: true
    nameserver: 127.0.0.1
    primaryrecord: "primary.registry-primary.service.consul."
    recordtype: SRV
    port: 8600

1. Run the migrations!

make bin/registry && ./bin/registry database migrate up ./myenv/config.yml
OK: applied 132 migrations in 3.778s