Ensure gaussholder meta data is not indexed

[roborourke]

Steps to reproduce:

1. Use gaussholder
2. Add some images
3. Enable document indexing
4. Index documents
5. Guassholder data URIs are in the search index

Acceptance criteria:

• Filter out all post metadata with the prefix gaussholder_ from search indexing using the ep_post_sync_args_post_prepare_meta on a late priority of 100

[veselala]

Hey team! Please add your planning poker estimate with ZenHub @robindevitt @roborourke @shadyvb

[robindevitt]

https://humanmade.com/2016/01/18/lightweight-image-previews-in-wordpress-with-gaussholder/

[veselala]

@roborourke please share some knowledge around this on our next refinement so that we can discuss the estimation

[roborourke]

There's a filter documented here: https://docs.altis-dxp.com/search/indexing/additional-data/

We need to make sure that any post meta keys with the prefix gaussholder_ are not indexed, updated the acceptance criteria accordingly.