[ssh signing] support custom signing program

[seanaye]

Describe the bug
When performing a commit I get the error
sign builder error: Failed to retrieve 'user.signingkey' from the git configuration: Currently, we only support a pair of ssh key in disk.

This is likely because of my git configuration for commit signing.

To Reproduce
Steps to reproduce the behavior:

1. Add the following to your ~/.gitconfig

[user]
...

[gpg]
	format = ssh
[gpg "ssh"]
	program = /Applications/1Password.app/Contents/MacOS/op-ssh-sign
[commit]
	gpgsign = true
[push]
	autoSetupRemote = true

2. try to perform a commit
3. See the error above

Expected behavior
This should work without error. git on the CLI works fine without error

Screenshots

Context (please complete the following information):

• M2 MacOS 14.4.1
• GitUI Version gitui nightly 2024-04-14
• Rust version: cargo 1.77.1 (e52e36006 2024-03-26)

Additional context
Add any other context about the problem here.

[extrawurst]

Yeah right now we do not support calling a custom ssh program as you have configured

[seanaye]

I am working on a PR to support this.