create multiple-line annotations for git tags

[skyfaller]

Is your feature request related to a problem? Please describe.
Happily we can now create annotations, and view annotations (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. 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.

[skyfaller]

Anyone who wants to use a tool like git-cliff for producing release notes and is not on GitHub may enjoy using long git annotations. This may become more popular as FOSS advocates are putting more effort into encouraging people to move off GitHub, and you can see projects like ForgeFed are getting funding from places like NLnet's User Operated Internet Fund to network the independent code forges together.

[extrawurst]

requires the same foundational functionality like #1171