Add support for Metadata field in all resources

[mthenw]

What did you implement:

Closes #435

metadata fields is map[string]string field that allows storing arbitrary key/value pairs and the filtering resources based on those values.

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 #470 into master will increase coverage by 0.79%.
The diff coverage is 94.11%.

@@            Coverage Diff            @@
##           master    #470      +/-   ##
=========================================
+ Coverage    68.5%   69.3%   +0.79%     
=========================================
  Files          34      35       +1     
  Lines        2137    2163      +26     
=========================================
+ Hits         1464    1499      +35     
+ Misses        612     602      -10     
- Partials       61      62       +1

Impacted Files	Coverage Δ
event/type.go	0% <ø> (ø)	⬆️
function/function.go	61.53% <100%> (+2.07%)	⬆️
metadata/metadata.go	100% <100%> (ø)
libkv/subscription.go	88.75% <100%> (+0.14%)	⬆️
libkv/function.go	85.18% <100%> (+0.37%)	⬆️
libkv/eventtype.go	86.04% <100%> (+0.33%)	⬆️
httpapi/httpapi.go	82.85% <100%> (+3.1%)	⬆️
libkv/cors.go	86.95% <33.33%> (-1.94%)	⬇️

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