Set default directory permissions on `~/.gcm/store` plaintext credential store

[mjcheetham]

Discussed in #393

^{Originally posted by BenAigan July 12, 2021}
Is it possible for the Git Credential Manager Core to create a directory with user only permissions rather than read and execute for the rest of the world especially if using plain text Credential Store?

Right now it's on the user to chmod the default directory ($HOME/.gcm/store) to 700, but it's a reasonable ask to have GCM create the directory with the appropriate permissions if it does not exist already (and not modify any existing directory).

[mjcheetham]

Implemented in #397