add "changed files" to pull-request webhook (similar to push webhook)

[6543]

as title

[anbraten]

It should contain all changed files since commit one of the PR, but remove changed files again if the change has been reverted again.

[6543]

for now if somebody like to get the info there is #21177

[anbraten]

Maybe in general a nicer approach to not contain those changes in the webhook payloads to not flood the receiver