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>

modules/structs/repo_file.go

@@ -4,6 +4,8 @@
 
 package structs
 
+import "time"
+
 // FileOptions options for all file APIs
 type FileOptions struct {
 	// message (optional) for the commit of this file. if not supplied, a default message will be used
@@ -121,6 +123,8 @@ type ContentsResponse struct {
 	Path          string `json:"path"`
 	SHA           string `json:"sha"`
 	LastCommitSHA string `json:"last_commit_sha"`
+	// swagger:strfmt date-time
+	LastCommitWhen time.Time `json:"last_commit_when"`
 	// `type` will be `file`, `dir`, `symlink`, or `submodule`
 	Type string `json:"type"`
 	Size int64  `json:"size"`