Entry metadata field

[crazy-max]

fixes #479
fixes #654

Provides new field {{ .Entry.Metadata }} that returns additional metadata with defaults depending on the provider. User can also add arbitrary metadata through diun.metadata.* labels/annotations like diun.metadata.foo=bar.

Example with mail notifier:

notif:
  mail:
    host: localhost
    port: 25
    ssl: false
    insecureSkipVerify: false
    from: diun@example.com
    to:
      - webmaster@example.com
    templateBody: |
      Docker tag {{ .Entry.Image }} which you subscribed to through {{ .Entry.Provider }} provider has been released.

      {{ range $key, $value := .Entry.Metadata }}
      * `{{ $key }}`: `{{ $value }}`
      {{- end }}

Webhook notifier:

{
  "diun_version": "4.24.0",
  "hostname": "myserver",
  "status": "new",
  "provider": "file",
  "image": "docker.io/crazymax/diun:latest",
  "hub_link": "https://hub.docker.com/r/crazymax/diun",
  "mime_type": "application/vnd.docker.distribution.manifest.list.v2+json",
  "digest": "sha256:216e3ae7de4ca8b553eb11ef7abda00651e79e537e85c46108284e5e91673e01",
  "created": "2020-03-26T12:23:56Z",
  "platform": "linux/amd64",
  "metadata": {
    "ctn_command": "diun serve",
    "ctn_createdat": "2022-12-29 10:22:15 +0100 CET",
    "ctn_id": "0dbd10e15b31add2c48856fd34451adabf50d276efa466fe19a8ef5fbd87ad7c",
    "ctn_names": "diun",
    "ctn_size": "0B",
    "ctn_state": "running",
    "ctn_status": "Up Less than a second (health: starting)"
  }
}