share.js incorrectly caches webforms with specific prepopulate fields

Problem/Motivation

I have a webform with a few fields that can be pre-populated. Using webform_share, I can embed the form with a URL like the following:

<script src="//www.example.net/webform/event_registration/share.js?variant=custom_opt_in&eventid=xxx&marketing_personas=General+Info;Event+Specific"></script>

This will then write the iframe element to the HTML document and correctly include the query parameters in the iframe src.

However, when the query parameters are changed in the script element, a cached version of the form is displayed, with the previous query parameters in the iframe src.

Steps to reproduce

• Create a webform with one or more fields set to '#prepopulate': true
• Do NOT configure the `form_prepopulate` setting to TRUE.
• Embed the script share code on another web page, adding a prepopulate query parameter to the src attribute (e.g. ?eventid=test)
• Load the hosting webpage to view the embedded form.
• Change the query parameter in the script src.
• Reload the hosting webpage and see the old prepopulated value in the embedded form field.

Looking at the source on a test form, I saw the following, and noticed that the eventid query parameter was different in the script and iframe src attributes:

<script src="//d9.test/webform/share_test/share.js?eventid=bar"></script>
<iframe src="http://d9.test/webform/share_test/share/iframe-resizer/4.2.10?eventid=foo" title="Share Test | Drush Site-Install" class="webform-share-iframe" allow="geolocation; microphone; camera" allowtransparency="true" allowfullscreen="true" style="width: 1px; min-width: 100%; overflow: hidden; height: 473px;" id="iFrameResizer0" scrolling="no" frameborder="0"></iframe>
...

Proposed resolution

Add some additional cache contexts to either the share.js response or the webform entity.