fix(firestore): Export Filter type from Firestore - #2167

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

Thank you @danReynolds for the contribution!
We need to update the api docs. Please do npm run api-extractor:local and push the new changes.

Please also update the unit and integration tests. For example:

I added a test and ran API extractor in #2192, so I think we can use that one instead. I set @danReynolds as co-author

#2192 has been merged! Thanks for starting this work, @danReynolds!