Adds py.typed to get type safe usage of modal_proto package #9533

Job	Run time
Unit tests on 3.8 and ubuntu-20.04 (protobuf=latest)	10m 50s
Unit tests on 3.9 and ubuntu-20.04 (protobuf=latest)	11m 16s
Unit tests on 3.10 and ubuntu-20.04 (protobuf=latest)	10m 46s
Unit tests on 3.11 and ubuntu-20.04 (protobuf=latest)	11m 57s
Check minimal container dependencies for 3.8 / 2024.04	27s
Unit tests on 3.12 and ubuntu-20.04 (protobuf=latest)	13m 40s
Check minimal container dependencies for 3.12 / 2024.04	29s
Unit tests on 3.10 and macos-12 (protobuf=latest)	16m 4s
Check minimal container dependencies for 3.8 / 2024.10	24s
Unit tests on 3.10 and macos-14 (protobuf=latest)	11m 7s
Check minimal container dependencies for 3.12 / 2024.10	22s
Unit tests on 3.10 and windows-latest (protobuf=latest)	18m 5s
Unit tests on 3.8 and ubuntu-20.04 (protobuf=3.19)	12m 59s
Publish base images for ${{ matrix.image-name }} ${{ matrix.image-builder-version }} on ${{ matrix.python-version }}	0s
Publish client package	0s
	1h 58m 26s

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