Use a public key for baggage values

Changed

• Baggage key is now a public string "logctx-data-map-string-interface" that can be set and read by anyone from any package.
  The type of that baggage will be always map[string]interface{}

Added

• BaggageContextKey constant that defines that string
• Baggage(ctx) map[string]interface{} that allows reading the underlying baggage.