Update development infrastructure

[JunTaoLuo]

Addresses #175. Most of the infrastructure is taken from aspnet/AspNetCore.

The get-dotnet.ps1/sh will now install locally to {repoRoot}.dotnet/ and the activate.sh/ps1 will set the appropriate environment variables to use the installed SDK. Edited README to reflect the new workflow.

[JunTaoLuo]

Hmm need to update https://github.com/grpc/grpc/blame/master/tools/dockerfile/interoptest/grpc_interop_aspnetcore/build_interop.sh#L27-L32 to fix the interop tests.

[JunTaoLuo]

Interop test is not required. I think there just needs to be an approval.

[jtattermusch]

Looks like this has broken the interop test build on grpc/grpc repository: grpc/grpc#18431
We need to be more careful when updating the way things are built, because grpc/grpc also runs a continuous interop test against master of grpc-dotnet. Basically whenever the script that runs interop tests in this repo is changing, same changes need to be done in grpc/grpc to avoid breakage.
(I would have probably anticipated this problem but this PR was merged without my LGTM)

[jtattermusch]

I might have spoken too soon - I didn't notice grpc/grpc#18714 earlier. But we should have synced more around merging this PR (and subsequently breaking grpc/grpc) anyway.

[JamesNK]

Sorry for creating issues with this PR. We underestimated the impact of changing the build in other places.

I think for now we'll always get your approval before merging, except if the PR has low risk/impact changes such as:

• A minor refactor or fix
• Unit test changes
• Sample or documentation

Does that sound good to you?

[jtattermusch]

Sorry for creating issues with this PR. We underestimated the impact of changing the build in other places.

I think for now we'll always get your approval before merging, except if the PR has low risk/impact changes such as:

• A minor refactor or fix
• Unit test changes
• Sample or documentation

Does that sound good to you?

SGTM.