feat: add SOPS_AGE_SSH_PRIVATE_KEY_CMD

[natescherer]

Hello! This PR adds support for a new environment variable SOPS_AGE_SSH_PRIVATE_KEY_CMD that functions similarly to SOPS_AGE_KEY_CMD. The value of this variable is executed and the output is used to derive an age key; the idea is for it to be used along with a password manager.

The README was updated with detail on this env var, tests were added that pass, and I also manually built and tested on macOS via make install.

Please let me know if you have any questions or would like anything changed.

Closes #1851.

[felixfontein]

Thanks for your contribution!

This likely introduces conflicts with #2009 (CC @gokselk), but since that PR is waiting for feedback from more people (and I have no idea how long that will take) I guess there's not much we can do about it here.

[natescherer]

@felixfontein I think everything should be up-to-date with your requests now. Please let me know if there's anything else you'd like me to change!

[natescherer]

Okay, should be up-to-date again (now that I removed the mise and vscode config I accidentally committed, sorry 😬).

[felixfontein]

Looks good! I have a few small improvements. Also I think it would be great to rebase and squash some of the commits (or even all).

Also could you rebase to latest main? Since I cannot push to your branch (and thus can't add a merge commit to change that), I won't be able to merge this unless this PR is based of the latest commit from main.

[natescherer]

Okay, I completed your latest round of revisions and squashed the whole thing down into a single commit! Should be up-to-date with main now, too.

[felixfontein]

@natescherer thanks for your contribution!

[natescherer]

@natescherer thanks for your contribution!

Thank you for accepting it and for your help improving the code!

[rkast]

@natescherer nice!