Add support for FieldPath for where-clause querying to Firestore Plugin.

[t-cun]

Querying Firestore doesn't yield results when the Field name consists special characters.
For example abc@gmail.com which consists '.'
Can you include a work around like including ` around the Field name so that it treats special characters as Normal one.

This issue was previously closed without any comments/updates. Re-opening as the issue still exists.

Reference:
flutter/flutter#22712
#626

[jyardin]

Is there any way to circumvent this problem?

[zephyo]

encountering this, does this have a fix yet?

[Ehesp]

You can use the FieldPath class to provide a path/key with special characters: https://github.com/FirebaseExtended/flutterfire/blob/29eb495bddafe3965e3b7f5f097d5122bf5e2d1a/packages/cloud_firestore/cloud_firestore_platform_interface/lib/src/field_path.dart#L7

Will close, but this should be documented.

[Ehesp]

Re-opening to track with testing

[Salakar]

Hey all, as part of our on-going work for #2582, this has been implemented in our Firebase Firestore rework (#2913) - which has now been merged into master. We'll look at publishing some prereleases in the next few days. Thank you