This PR adds support for entering your git GPG passphrase through the
askpass UI. As a daily Zed user I'm really missing this feature, this
forces me to switch to a terminal or an external application. which is
not ideal for me and most people that are forced to use GPG signing.

Right now Zed can show a similar error (redacted) when you are trying to
commit with GPG signing enabled:
```
error: gpg failed to sign the data:
[GNUPG:] KEY_CONSIDERED <redacted> 2
[GNUPG:] BEGIN_SIGNING H8
[GNUPG:] PINENTRY_LAUNCHED 4625 curses 1.3.2 - xterm-ghostty - - 501/20 0
gpg: signing failed: Inappropriate ioctl for device
[GNUPG:] FAILURE sign <redacted>
gpg: signing failed: Inappropriate ioctl for device

fatal: failed to write commit object
```

Here is the key configurion parts of my **global** git config:

```
[user]
	name = Remco Smits
	email = <redacted>
	signingkey = <redacted>
[commit]
	gpgsign = true
[tag]
	gpgsign = true
```

**Before**


https://github.com/user-attachments/assets/60a06574-92f1-45df-a29a-8ae11db6751d

**After**


https://github.com/user-attachments/assets/92b648e9-b538-4ce5-af4c-136689d14e1a


**How to test this?**
1. Create a key `gpg --full-generate-key`
2. Run `git config --global user.signingkey <KEY ID>`
3. Run `git config --global commit.gpgsign true`
4. Run & copy result from `gpg --armor --export <KEY ID>` and submit
your public key to github
[https://github.com/settings/gpg/new](https://github.com/settings/gpg/new)
5. Make a change and try committing
6. See that it promts for your passphrase :)

**Note**: I used AI as an assistant to write this PR

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Git: Passphrase prompts from GPG to unlock commit signing keys are now
shown in Zed.

Co-authored-by: Lukas Wirth <lukas@zed.dev>