Load plugin secrets if present

[aduane]

https://canvasmedical.atlassian.net/browse/KOALA-1333

Looks for a SECRETS.json in a plugin's root directory and loads it if present. Protocols will be initialized with the secrets dict, so they can be accessed in protocol code. Example:

class MyCoolProtocol(BaseProtocol):
    def compute(event: Event):
        http.post(self.secrets['BASE_URL'])
        return [Effect(...)]

Oh and I also deleted all the plugins in custom-plugins/. We don't want those to accidentally end up installed somewhere.

[beaugunderson]

can you move one of the custom plugins to a different directory? super useful for testing the installation stuff, and would be useful for unit tests

[aduane]

@beaugunderson done