Add new s2s config option

[thomas-33across]

Type of change

• Feature
• Does this change affect user-facing APIs or examples documented on http://prebid.org?

Description of change

Adding a new option in the s2s configuration, which allow bidder to modify the sync url before the actual call to the sync endpoint. This enable bidder to have customization over the url, and also enable them to add custom params

• new s2s configuration options

{
  bidders: ['33across'],
  adapter: 'prebidServer',
  syncEndpoint: 'http://www.mylocal.com:8000/cookie_sync',
  syncUrlModifier: {
    '33across': function(type, url, bidder) {
      const publisherId = '00000123231231'
      url += `&ri=${publisherId}`;

      return url
    }
  }
}

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/
  update s2s config params prebid.github.io#1216

[thomas-33across]

Hello Prebid team,
I will do the PR for the docs if this propose change can be done.

Thank you

[bretg]

I'm empathetic with this hook - we had a similar issue resolved by relying on the s2sConfig.accountId, but that doesn't work if we're not the host.

Assigning to @mkendall07 to approve the API change.

[bretg]

Should this be testing for syncUrlModifier before attempting to index into bidder?

i.e.

if (_s2sConfig.syncUrlModifier and typeof _s2sConfig.syncUrlModifier[bidder] === 'function')

[mkendall07]

This has the slight disadvantage of making the s2s config not JSON configurable. If folks are ok with that tradeoff I don't mind approving.

[bretg]

Rubicon is ok with this approach.

(thomas-33across - thanks for adding the object check)

[mkendall07]

This LGTM. We just need a documentation PR and then can merge.

[thomas-33across]

Hello prebid team
I had create a new PR for the doc and here is the link:
prebid/prebid.github.io#1216

Thank you

[bretg]

Merging.