Allow spaces in collection names

[BrunoRoemers]

Expected Behavior

ODM can handle spaces in collection ids. The official restrictions on collection ids are documented here: https://firebase.google.com/docs/firestore/quotas#limits

Actual Behavior

ODM refuses to generate code when a collection id containing a space is passed to @Collection().

Relevant LOCs: https://github.com/FirebaseExtended/firestoreodm-flutter/blob/main/packages/cloud_firestore_odm_generator/lib/src/collection_data.dart#L341

Unfortunately we have a few collections with spaces in their names, which blocks us from fully adopting the Cloud Firestore ODM.

Hacky Workaround

1. Temporarily remove the space of the collection name; i.e. @Collection('hello world') becomes @Collection('helloworld')
2. Run code generator (works)
3. Edit the generated file; add the space again

This is a maintenance nightmare though.

[rrousselGit]

Sure. Fancy making a PR?