feat(cloud_firestore_odm): Add support for DateTime/Timestamp

[rrousselGit]

Description

Blocked by google/json_serializable.dart#1135

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[github-actions]

Visit the preview URL for this PR (updated for commit 19e94e3):

https://flutter-firebase-docs--pr8563-rrousselgit-odm-tim-x8z405jl.web.app

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[wstrange]

Now that the json blocker has been merged (I think?) can this PR proceed?

[rrousselGit]

The blocker isn't json_serializable.