Jitsu SDK should support sources

[vklimontovich]

Summary

We are going to implement Connector SDK as part of Jitsu SDK. The SDK should provide an easy way to develop jitsu connectors (aka sources) in TypeScript:

• jitsu source create - should create an empty project
• jitsu source build - should build a source file
• jitsu source test [--config-file file] - should run a test (or set of tests)

The implementation phases:

• [ ] Implement the first version of API. API should be similar to Go core API
• [ ] Implement first source and tests for it - Airtable and Redis

Implementing Go-bindings is out of scope of this issue