Implement support for multi-site data

[lexisother]

This PR implements very basic support for configuring a View to support retrieving data from multiple sites.

Currently this only supports environments where the root URL of all Craft sites is the same. (think of $PRIMARY_SITE_URL/site1, $PRIMARY_SITE_URL/site2, etc)
This because a Source can only be configured with one property/app, which are (typically) only for one single domain.

However, support for differing root domains can be added by adjusting the payload I added (and making whatever other necessary changes within Metrix):

"dimensionFilter": {
  "filter": {
    "fieldName": "pageLocation",
    "stringFilter": {
      "matchType": "BEGINS_WITH",
      "value": "https://example.com/"
    }
  }
}

There's no need to merge this PR as-is, or at all, at the very least this is a reference implementation for something that could be. For now, I will likely be running a fork of Metrix for a client that requested this functionality.

[bramnijssen]

Addresses #2

[engram-design]

Thanks for this, appreciate it! I'll give it a look over to see if practical to merge, or if we want to tweak further for release.