Event types

[mthenw]

What did you implement:

This PR introduces Event Type concept to EG. It allows registering event types that can be consumed by function in a space.

Event types API is an API that allows defining event types on per space basis. Every event emitted to a space has to have type registered beforehand. If there is no event type registered an error is returned by Events API.

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: YES

[codecov]

Codecov Report

Merging #433 into master will increase coverage by 1.48%.
The diff coverage is 76.3%.

@@            Coverage Diff             @@
##           master     #433      +/-   ##
==========================================
+ Coverage   63.89%   65.38%   +1.48%     
==========================================
  Files          29       32       +3     
  Lines        1601     1788     +187     
==========================================
+ Hits         1023     1169     +146     
- Misses        536      573      +37     
- Partials       42       46       +4

Impacted Files	Coverage Δ
httpapi/config.go	0% <0%> (ø)	⬆️
event/type.go	0% <0%> (ø)
event/errors.go	0% <0%> (ø)	⬆️
internal/cache/target.go	0% <0%> (ø)	⬆️
internal/cache/subscription_cache.go	86.15% <100%> (ø)	⬆️
libkv/function.go	82.27% <100%> (-0.86%)	⬇️
httpapi/metrics.go	100% <100%> (ø)	⬆️
event/event.go	77.41% <100%> (ø)	⬆️
libkv/subscription.go	88.09% <100%> (+0.21%)	⬆️
internal/cache/eventtype_cache.go	100% <100%> (ø)
... and 6 more

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[RaeesBhatti]

Looks good to me 👍