objectsigner/auto: New package to return a `signer` based on materialised Git config

[pjbgf]

Feature Description

Some applications use go-git to generate commits against a local repository on behalf of the current user.
A new ObjectSigner auto should use the same heuristics as git to identify what type of signer (and which key) should be used for a given context, without requiring the use of go-git primitives - to avoid circular reference.

Initially support can be added for gpg and ssh, as both object signers already exist. As new implementations are added, them should make their way into auto.

Relates to: go-git/go-git#1849