support omitted streams

[Alex-Bair]

The activity_feed stream in source-greenhouse is an array API response that should be flattened, but the Airbyte connector emits it as-is. An abbreviated example response is below:

{
  "notes": [
    {
      "id": 1,
    }
  ],
  "emails": [
    {
      "id": 1,
    }
  ],
  "activities": [
    {
      "id": 1,
    },
    {
      "id": 2,
    }
  ]
}

The real solution is to fix the connector so individual objects from the response arrays are emitted instead of emitting the entire response as as single document. However, until the connector is fixed, omitting activity_feed from discovery is a temporary fix.

We previously had the capability to omit streams from discovery with selected_streams.json, but we changed it to specify streams that should be enabled & disabled everything else. To re-add the capability to completely omit streams from discovery, this PR adds omitted_streams.json.

I tested this locally with source-greenhouse, and activity_feed is omitted from discovery.