Bug 1693486 - New event extra properties

[badboy]

So far this is an early draft that implements the new calling convention in Kotlin and Swift.
Stuff to do before this is ready though.

• Implement it in glean-parser
• Decide on the name of the new interface/protocol
• Prototype the C++/JavaScript API for FOG
• Python implementation (might be split out into a followup)
• Kotlin
  • Connection between the enum <-> interface, so the EventMetricType is generic over the right thing (necessary to avoid using the some extras on the wrong event)
• Swift
  • Connection between the enum <-> protocol, so the EventMetricType is generic over the right thing (necessary to avoid using the some extras on the wrong event)

[badboy]

Only just now realized we're also changing the payload format. That's ... more work

[badboy]

Turns out it's a bit more complicated than initially sketched out.
We also want to change the payload, which means we can't do the $type -> string conversion on the language binding side, but need to pass in the right values.
So we need to separate out the two APIs to handle them differently.

[badboy]

Blocked by the discussion in https://bugzilla.mozilla.org/show_bug.cgi?id=1693486

[badboy]

Closing this in favor of only changing the user-facing API (which will probably include this code, but let's start with a fresh PR)