[github plugin] split the field github.repo

[Issif]

What type of PR is this?

Uncomment one (or more) /kind <> lines:

/kind bug

/kind cleanup

/kind design

/kind documentation

/kind failing-test

/kind feature

Any specific area of the project related to this PR?

Uncomment one (or more) /area <> lines:

/area plugins

/area registry

/area build

/area documentation

What this PR does / why we need it:

Split the field github.repo into github.repo.name and github.repo.url

Which issue(s) this PR fixes:

#300

Fixes #

Special notes for your reviewer:

I didn't updated the changelog, when i use the function in the makefile, it breaks the file, what's the good way to process?