Allow additional omnisharp settings to be overridden at the workspace level

Currently `omnisharp.path` and `omnisharp.useGlobalMono` cannot be overridden at the workspace level.

Reason for change:

Currently, due to an architectural issue in roslyn, omnisharp's scripting support does not support custom scripting hosts. Because of this a custom omnisharp build must be used in order to use a custom scripting host. The problem with that is the omnisharp implementation is global.

Allowing `omnisharp.path` would allow the omnisharp implementation to be overridden only where needed.

The reason for needing `omnisharp.useGlobalMono` is because setting it to `never` is the only way to run a custom omnisharp build without mono. My current omnisharp build doesn't use mono (.net 5 instead).

Having these two settings available at the workspace level would allow the vscode extension to be used in custom scripting scenarios without affecting the normal operation machine wide.