[Feature request] Signing method and autosign on commit

[Leny1996]

Sign option is great, but sometimes I need to change my local config (gpg or ssh key signing) and sometimes I don't need to sign at all.
It would be a great option to have this configured using git-user-switch (no more additional git commands).

Github docs explaining signing methods

What we want to achieve is to add option to enable commit.gpgsign and gpg.format (additionaly gpg.x509.program when using gpg.format x509).

I hope everything is clear.