Index Exchange: Registering user syncs

[ix-prebid-support]

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

• Added user sync callback for Index Exchange

Sample test config

pbjs.setConfig({
    userSync: {
        iframeEnabled: true,
        filterSettings: {
            iframe: {
                bidders: ['ix'],
                filter: 'include'
            }
        }
    }
});

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

• contact email of the adapter’s maintainer
• 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

[ix-prebid-support]

The failed unit test (in sortableAnalyticsAdapter_spec.js) is not related to the changes in this PR.

[musikele]

LGTM, but I'm waiting for circleci build to pass.