create multiple-line annotations for git tags

**Is your feature request related to a problem? Please describe.**
Happily we can now [create annotations](https://github.com/extrawurst/gitui/issues/747), and [view annotations](https://github.com/extrawurst/gitui/issues/1076) (including longer multiple-line annotations). Thank you for making that happen!

Sadly, if you want to create an annotation that takes up more than one line, gitui currently can't do it. I need this for my use case, which is [putting release notes in git tag annotations](https://drewdevault.com/2021/05/19/How-to-write-release-notes.html). This is helpful for bringing release notes to non-GitHub platforms and local repos. Without this, I will still be stuck creating annotations from git itself.

When I go to create a tag in gitui, I hit `t` to write the tag name, then `^a` to write the annotation. But here enter/return would save the annotation, there is no way to write a second line.

**Describe the solution you'd like**
Given the current workflow, I think the simplest solution would be to add `^e` to open the editor, as we do when writing a commit message.

Inizia dal workflow di creazione dei tag descritto nell’issue e confrontalo con il flusso esistente dell’editor dei messaggi di commit. Aggiungi un modo per aprire quell’editor durante l’inserimento di un’annotazione del tag, quindi verifica che il Git tag risultante contenga più righe di annotazione.

Scritto dal modello di indicizzazione a partire dal testo della issue.