github: add code coverage with codecov.io

[arvindbr8]

Fixes: #1676

Codecov Reach graph - https://app.codecov.io/github/grpc/grpc-go/tree/code_coverage

RELEASE NOTES: none

[codecov-commenter]

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[arvindbr8]

Discussed offline. I've made a few changes since the last push.

We had a cumulative problem of go test --cover only covering unit tests. It did not include coverage from e2e style tests. For this adding coverpkg=./... worked which includes coverage of tests

Since most of the changes are to the main module, it makes sense to limit the scope of presubmit check to only run go test --cover for the main module.

Ignored the following paths in codecov.

• "test/"
• "benchmark/"
• "internal/testutils"
• ".*.pb.go"