Support `Message.from_dict()` as a class and an instance method (#476)

* Make Message.from_dict() a class method

Signed-off-by: Marek Pikuła <marek.pikula@embevity.com>

* Sync 1/2 of review comments

* Sync other half

* Update .pre-commit-config.yaml

* Update __init__.py

* Update utils.py

* Update src/betterproto/__init__.py

* Update .pre-commit-config.yaml

* Update __init__.py

* Update utils.py

* Fix CI again

* Fix failing formatting

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Signed-off-by: Marek Pikuła <marek.pikula@embevity.com>
Co-authored-by: James Hilton-Balfe <gobot1234yt@gmail.com>