Add Topics to PushEventRepository

[ecrupper]

I would like to leverage the topics field found in the repository object of several webhook event payloads. I noticed that for the push event, which has a special repo struct, the Topics field was missing.

I plan on implementing this enhancement the same way the standard repository struct does it, unless there are any objections. I am curious why it is of type []string and not *[]string though, if anyone would be willing to clarify that for me? It doesn't get an accessor generated as a result.

[gmlewis]

We typically only use a pointer to a slice when it is important in the GitHub v3 API to accurately distinguish between when the slice is provided and when it is not. Otherwise, we typically use just a slice since they are easier to deal with.

Yes, PRs to add missing fields are welcome, thank you!