[Feature Request] Firestore SetOptions.ignoreFields()

## What feature would you like to see?

We already have `mergeFields()`, it will be better to also have `ignoreFields()`. A possible use case would be when our data has a lot of fields, especially deep, nested, and dynamic ones which will be impossible to use `mergeFields`, while we just want to ignore one or two fields.

## How would you use it?

Just the same as `mergeFields()`, it is just the opposite of it.