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_get_contents_test.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"net/url"
 	"testing"
+	"time"
 
 	auth_model "forgejo.org/models/auth"
 	repo_model "forgejo.org/models/repo"
@@ -33,18 +34,19 @@ func getExpectedContentsResponseForContents(ref, refType, lastCommitSHA string)
 	gitURL := setting.AppURL + "api/v1/repos/user2/repo1/git/blobs/" + sha
 	downloadURL := setting.AppURL + "user2/repo1/raw/" + refType + "/" + ref + "/" + treePath
 	return &api.ContentsResponse{
-		Name:          treePath,
-		Path:          treePath,
-		SHA:           sha,
-		LastCommitSHA: lastCommitSHA,
-		Type:          "file",
-		Size:          30,
-		Encoding:      &encoding,
-		Content:       &content,
-		URL:           &selfURL,
-		HTMLURL:       &htmlURL,
-		GitURL:        &gitURL,
-		DownloadURL:   &downloadURL,
+		Name:           treePath,
+		Path:           treePath,
+		SHA:            sha,
+		LastCommitSHA:  lastCommitSHA,
+		LastCommitWhen: time.Date(2017, time.March, 19, 16, 47, 59, 0, time.FixedZone("", -14400)),
+		Type:           "file",
+		Size:           30,
+		Encoding:       &encoding,
+		Content:        &content,
+		URL:            &selfURL,
+		HTMLURL:        &htmlURL,
+		GitURL:         &gitURL,
+		DownloadURL:    &downloadURL,
 		Links: &api.FileLinksResponse{
 			Self:    &selfURL,
 			GitURL:  &gitURL,