Add endpoint for updating event type

[mthenw]

What did you implement:

Updating event type

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?: NO

[codecov]

Codecov Report

Merging #455 into master will increase coverage by 1.37%.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##           master     #455      +/-   ##
==========================================
+ Coverage   64.74%   66.11%   +1.37%     
==========================================
  Files          32       32              
  Lines        1835     1883      +48     
==========================================
+ Hits         1188     1245      +57     
+ Misses        596      585      -11     
- Partials       51       53       +2

Impacted Files	Coverage Δ
event/errors.go	0% <0%> (ø)	⬆️
function/errors.go	0% <0%> (ø)	⬆️
httpapi/httpapi.go	73.13% <100%> (+8.13%)	⬆️
libkv/subscription.go	88.02% <50%> (-0.08%)	⬇️
libkv/function.go	84.81% <50%> (ø)	⬆️
libkv/eventtype.go	85.71% <78.26%> (-2.17%)	⬇️

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