SharedIdSystem: add configurable inserter

[dgirardi]

Type of change

• Feature

Description of change

This adds the option to set inserter in the EIDs generated by sharedId, taking it from params.inserter, e.g.

pbjs.setConfig({
  userSync: {
    userIds: [{
      name: 'sharedId',
      params: {
        inserter: 'publisher.com'
      },
    }]
  }
});

Other information

Closes #12641