Add sentry messages ingestion path

Refers to #2009 (closed)

Added support for Sentry Message data items.

Scope of this MR:

• We store only message events
• A message can be stored using the Store endpoint. This behaviour can be tested using the GO, Python and JS SDK examples.
• A message can also be stored using the Envelope endpoint. This behaviour can be tested using the Ruby SDK example.
• Message is supported also for client side buffering
• Refactor code so that we can extend it easily in the future as new data items comes in
• Currently messages will not support stacktraces. Created #2040 (closed) to add this functionality on a later iteration.