IX Bid Adapter: Add support for IMG based user syncs

[ccorbo]

Type of change

• Bugfix
• Feature
• New bidder adapter
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Does this change affect user-facing APIs or examples documented on http://prebid.org?
• Other

Description of change

This change adds IMG based user sync support for the IX Bid Adapter.

• test parameters for validating bids

{
  bidder: '<bidder name>',
  params: {
    // ...
  }
}

Be sure to test the integration with your adserver using the Hello World sample page.

• [email protected]
• official adapter submission

For any changes that affect user-facing APIs or example code documented on http://prebid.org, please provide:

• A link to a PR on the docs repo at https://github.com/prebid/prebid.github.io/

Other information

[musikele]

LGTM