[cloud_firestore] implementation of FieldPath.of()

[Reprevise]

Bug report

Describe the bug
I'm trying to update a field that has special characters in its name. The error message says to use FieldPath.of() which does not exist.

Error message:

Use FieldPath.of() for field names containing '~*/[]'.)

I've referenced this answer from a Firebase engineer who says it's currently not possible to query/update a field that contains special characters since there's no wrapper for FieldPath.of()

Steps to reproduce

Steps to reproduce the behavior:

1. Create/update a field that contains special characters (~*/[])
2. See error or incorrect behavior

Expected behavior

The field should be created/updated.

[alextat120]

Can we also support FieldPath in the update method like in JS?

#5057