Feature: Add query builder for Campaign Donations

[kjohnson]

Related GIVE-1221

Description

This PR adds a query builder for campaign donations, which are aggregated from campaign related donation forms.

$query = new CampaignDonationQuery($campaign);
$query->sumIntendedAmount();

$query = new CampaignDonationQuery($campaign);
$query->countDonations();

$query = new CampaignDonationQuery($campaign);
$query->coundDonors();

Pre-review Checklist

• Relevant @unreleased tags included in DocBlocks
• Includes unit tests
• Self Review of code and UX completed

[alaca]

Looks good 👍