[TRI-36] Add connections to the SDK

[ericallam]

A connection is a new concept in the SDK. Connections can have the following functionality:

• A set of tasks
• Authentication information
• An exposed "client" that already has authentication but will still need to be wrapped in a task
• Events

_TRI-36

[ericallam]

Connections should also be used to authenticate "external sources", which is another concept we need to introduce into the SDK. External sources are things like Webhooks, and the "ExternalSource" in a specific integration can reuse the connection concept to register and unregister the webhook, as well as handle requests from webhooks