Databases: Add online migration support

[danaelhe]

Add resource digitalocean_database_online_migration to satisfy github issue #1287

Tests passing:

--- PASS: TestAccDigitalOceanDatabaseOnlineMigration_Basic (444.66s)
PASS
ok  	github.com/digitalocean/terraform-provider-digitalocean/digitalocean/database	(cached)

An example config:

resource "digitalocean_database_cluster" "source" {
  name       = "st01"
  engine     = "mysql"
  version    = "8"
  size       = "db-s-1vcpu-1gb"
  region     = "nyc1"
  node_count = 1
  tags       = ["production"]
}

resource "digitalocean_database_cluster" "destination" {
  name       = "dt01"
  engine     = "mysql"
  version    = "8"
  size       = "db-s-1vcpu-1gb"
  region     = "nyc1"
  node_count = 1
  tags       = ["production"]
}

resource "digitalocean_database_db" "source_db" {
  cluster_id = digitalocean_database_cluster.source.id
  name       = "terraform-db-om-source"
}

resource "digitalocean_database_online_migration" "foobar" {
  cluster_id = digitalocean_database_cluster.destination.id
  source {
    host = digitalocean_database_cluster.source.host
    db_name = digitalocean_database_db.source_db.name
    port = digitalocean_database_cluster.source.port
    username = digitalocean_database_cluster.source.user
    password = digitalocean_database_cluster.source.password
  }
  depends_on = [digitalocean_database_cluster.destination, digitalocean_database_cluster.source, digitalocean_database_db.source_db]
}