Update .NET SDK to 1.3.1 and add proxy tests #1106

Job	Run time
build-python (ubuntu-latest)	24s
build-java (ubuntu-latest)	27s
build-go (ubuntu-latest)	35s
build-dotnet (ubuntu-latest)	12s
build-ts (ubuntu-latest)	20s
feature-tests-ts / test	1m 58s
feature-tests-java / test	1m 49s
feature-tests-python / test	1m 25s
feature-tests-go / test	1m 34s
feature-tests-dotnet / test	1m 25s
build-docker-images / build-dotnet-docker-images / Build cs docker image	2m 31s
build-docker-images / build-go-docker-images / Build go docker image	4m 45s
build-docker-images / build-java-docker-images / Build java docker image	3m 54s
build-docker-images / build-python-docker-images / Build py docker image	3m 26s
build-docker-images / build-ts-docker-images / Build ts docker image	4m 18s
	29m 3s

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