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.

Beginne mit dem im Issue beschriebenen Workflow zur Tag-Erstellung und vergleiche ihn mit dem bestehenden Ablauf des Commit-Message-Editors. Füge eine Möglichkeit hinzu, diesen Editor beim Eingeben einer Tag-Annotation zu öffnen, und überprüfe anschließend, dass der resultierende Git-Tag mehrere Zeilen in der Annotation enthält.

Vom Indexierungsmodell aus dem Issue-Text verfasst.