Send placeholders for configured native assets

[matthewlane]

Type of change

• Feature

Description of change

Transmitting some types of native assets via querystring parameters can cause issues. For example, large strings may exceed an adserver's url call limit, and special characters can get mangled. This PR introduces a configuration option to send placeholder values in adserver targeting, instead of asset values. The native creative template can then replace the placeholder values with the actual values through a postmessage request to Prebid (a PR to the native functionality in prebid-universal-creative will do this automatically).

Given the configuration:

pbjs.addAdUnits({
  code: slot.code,

  mediaTypes: {
    native: {
      body: {
        required: false,
        sendId: true
      },
      clickUrl: {
        required: true,
        sendId: true
      },
    },
  },

  bids: [...]
});

The body and clickUrl assets will be sent as placeholder values hb_native_body:<adId> and hb_native_linkurl:<adId>. All other native assets will be sent normally.

[mkendall07]

LGTM

[jsnellbaker]

Largely LGTM; just made a note about the new field name in the native mediaTypes.

Also we need to get the docs changes put together.

[jsnellbaker]

Any new thoughts on the name for this property? I know we were throwing other ideas around in a different discussion.

cc @mkendall07

[mkendall07]

yeah good call on docs. @matthewlane can you get the doc PR up today?

[matthewlane]

Docs PR: prebid/prebid.github.io#1175

[mkendall07]

LGTM

[jsnellbaker]

LGTM