Add ProjectsV2Item event payload

[gsmet]

Frankly, this thing is underdocumented and IMHO not very usable but I
suppose it will improve over time.
Someone asked for it in my Quarkus GitHub App project so I thought I
might as well implement it.

@bitwiseman I think you relaxed the rules about documenting the simple getters but I'll add javadoc if required.

[codecov]

Codecov Report

Merging #1502 (5e26d18) into main (08785e5) will increase coverage by 0.07%.
The diff coverage is 96.29%.

@@             Coverage Diff              @@
##               main    #1502      +/-   ##
============================================
+ Coverage     78.80%   78.88%   +0.07%     
- Complexity     2113     2123      +10     
============================================
  Files           202      204       +2     
  Lines          6427     6454      +27     
  Branches        361      361              
============================================
+ Hits           5065     5091      +26     
- Misses         1152     1153       +1     
  Partials        210      210              

Impacted Files	Coverage Δ
...main/java/org/kohsuke/github/GHProjectsV2Item.java	88.88% <88.88%> (ø)
...c/main/java/org/kohsuke/github/GHEventPayload.java	80.00% <100.00%> (+0.23%)	⬆️
...va/org/kohsuke/github/GHProjectsV2ItemChanges.java	100.00% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[bitwiseman]

@gsmet

Frankly, this thing is underdocumented and IMHO not very usable but I suppose it will improve over time.

I suppose you mean the underlying API, not this PR, right?

[gsmet]

Yes. The PR is fine given what’s in the payload. We can’t do much better.

There might be some things that are not covered as I had to experiment to understand what were in the changes. But it should be resilient enough to not break in this case.