models/asymkey: Implement Tag verification

This is, in large part, a refactoring: we rename `CommitVerification` to
`ObjectVerification`, and adjust `ParseObjectWithSignature` (previously
`ParseCommitWithSignature`) to work on an object, rather than a commit.

This in turn, lets us implement `ParseTagWithSignature` on top of it, so
commit & tag signature verification will share most of the code.

Work sponsored by @glts.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

modules/gitgraph/graph_models.go

@@ -238,7 +238,7 @@ func newRefsFromRefNames(refNames []byte) []git.Reference {
 type Commit struct {
 	Commit       *git.Commit
 	User         *user_model.User
-	Verification *asymkey_model.CommitVerification
+	Verification *asymkey_model.ObjectVerification
 	Status       *git_model.CommitStatus
 	Flow         int64
 	Row          int