Add an extension configuration option to allow use of credential file instead of keyring on all platforms.

[sagerb]

For debugging and testing, we need a way of testing the credential file usage on platforms that have working keyring support. The easiest/cleanest way to implement this is to add an extension configuration option. This would seem to be a good example of something that users might also want to use, so I'm recommending we expose it publically through VSCode's configuration facility.