feat(api): add last_commit_when to contents response (#7418)

- Add a new field `last_commit_when` to the `ContentResponse` type, which is populated with the last commit's commiter date. This can be used to determine when the last edit of the content was.
- This field is compatible with what Gitea will likely add, https://github.com/go-gitea/gitea/pull/32921. There's no field for this information in the Github API, so no way to be compatible with that (this API endpoint is otherwise fully compatible with Github's API).
- Ref: gitnex/GitNex#1225
- Integration test adjusted. The API tests cannot test the actual output, as `testify` tries to 'deep equal' the `time.Time` structs which will differ due how the `time.Time` struct is created. Unit tests still verify the output.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7418
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

tests/integration/api_repo_files_change_test.go

@@ -104,6 +104,10 @@ func TestAPIChangeFiles(t *testing.T) {
 			var filesResponse api.FilesResponse
 			DecodeJSON(t, resp, &filesResponse)
 
+			// Testify cannot assert time.Time correctly.
+			expectedCreateFileResponse.Content.LastCommitWhen = filesResponse.Files[0].LastCommitWhen
+			expectedUpdateFileResponse.Content.LastCommitWhen = filesResponse.Files[1].LastCommitWhen
+
 			// check create file
 			assert.Equal(t, expectedCreateFileResponse.Content, filesResponse.Files[0])