(#4) Added option to Include File URL in schema.

[KyleSmith0905]

In response to the issue I made (#4):

I added a potential option that allows the File URLS to be removed from the schema, the Firebase DAM, for example, came with an authorization token in the URL that allows anyone to view it despite the rules set. I've only edited the Firebase DAM to have the option to include File URL.

I couldn't test this on a running sanity website, though I plan to try on Monday.

This is a small, non-breaking change.

I'm open for suggestions. Also, I should also add a guide on README.md about the new option.