(change) Make Slack, Discord and Teams change notifications readable #6169

Motivation

The Slack, Discord and Microsoft Teams notifiers render each changed field with render.Render, which prints Go internals rather than values. A rule added through a flag file currently arrives in Slack as:

Rules: nil => (*[]flag.Rule){flag.Rule{Name:(*string)("Test Rule"), Query:(*string)("biddableId eq \"123903\""), VariationResult:(*string)("on"), Percentages:(*map[string]float64)(nil), ProgressiveRollout:(*flag.ProgressiveRollout)(nil), Disable:(*bool)(nil)}}

Dates are worse, since time.Time is printed as its internal fields ({wall:0, ext:63926787600, loc:...}).

Separately, the three notifiers only keep changes of type update, so a field that is added or removed is silently dropped. Adding a new metadata key or a new variation produces a "Flag updated" message with no details.

Requirements

• Render changed values as JSON, using the existing json tags on the flag structs, so pointers and time internals disappear:

  Rules: null => [{"name":"Test Rule","query":"biddableId eq \"123903\"","variation":"on"}]
  Experimentation: null => {"start":"2026-10-05T09:00:00Z","end":"2026-10-12T09:00:00Z"}
  Disable: null => true
  

• Fall back to the current rendering for any value JSON cannot encode.
• Include fields that were added or removed, not only updated ones.
• Share one implementation across the Slack, Discord and Teams notifiers.
• No new dependency and no configuration change. Message layout and field paths stay as they are.

I have a PR ready for this and am happy to adjust it.