Add support for Linux enterprise deployable default settings

[mjcheetham]

Mirroring #1811, add support to set default settings for Linux, via MDM tooling. For this platform we look for files in the /etc/git-credential-manager/config.d directory.

Files are a simple format of key=value\n, where the key is a configuration value name from docs/config.md. This format prevents values from containing a \n line feed character, but this is unlikely to be an issue in practice.