RPO-2012: Add check for shared id

[BrettBlox]

Adds check for sharedId to satisfy comments made on this PR to prebid.
Falls back to our original functionality of checking for vmconcert_id in local storage or creating/setting one there if it does not yet exist.

Partners will need to update their Prebid configuration to add something similar to this if they wish to use sharedId:

pbjs.setConfig({
  userSync: {
    userIds: [
      {
        name: "sharedId",
        storage: {
          type: "cookie",
          name: "_sharedid", // create a cookie with this name
          expires: 365, // expires in 1 years
        },
      },
    ],
  },
});

and also add the sharedIdSystem submodule to their build:

$ gulp build --modules=concertBidAdapter,concertAnalyticsAdapter,sharedIdSystem

[andrew-a-dev]

👍 Nice addition!