Require CloudEvents Source be a URI

[alexdebrie]

What did you implement:

Changed the validation requirement on the Source field of the Event struct to require a URI, rather than a URL. This is according to the CloudEvents spec.

How did you implement it:

Changed validation on Source from validate:"url,required" to validate:"uri,required".

I also changed the test that checks that a valid CloudEvent is not transformed into a CloudEvent. I changed the test case's source to be a URI rather than a URL so that we would catch this in the future.

How can we verify it:

Emit an event in CloudEvents format that uses a URI (not URL) in the source property. Make sure that it wasn't transformed into a CloudEvent.

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 #414 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #414   +/-   ##
=======================================
  Coverage   63.85%   63.85%           
=======================================
  Files          29       29           
  Lines        1649     1649           
=======================================
  Hits         1053     1053           
  Misses        553      553           
  Partials       43       43

Impacted Files	Coverage Δ
event/event.go	69.35% <ø> (ø)	⬆️

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