Need to support multiple workflows running in a single APP

[VictorS67]

Current encre.app.config.js file only support single workflow path in 'workflow' attribute, however, we should support multiple workflows running in a single App.

To do this, we need to change the schema of the 'workflow' attribute. Can change it to either string or string[].

I don't think we need to refactor anything else, except we should let users know what are the required attributes for configurating handlers. For example, if multiple workflows is configured, then every handler should have non-empty attribute 'graphId' to indicate which workflow it is belong to. Thus, a validation step to check workflow schema is required here.