Add appinsights telemetry

[qmuntal]

This PR makes all go executions upload telemetry data to Azure Application Insights as an opt-in feature which requires manually setting an instrumentation key. Therefore is safe to merge into main until we pass the privacy review.

Set this env variable to enable telemetry on your computed:

MS_GOTOOLCHAIN_TELEMETRY_INSTRUMENTATION_KEY=x # here you need a real instrumentation key!
MS_GOTOOLCHAIN_TELEMETRY_ALLOW_GO_DEVEL=1
MS_GOTOOLCHAIN_TELEMETRY_STATS=1 # optional, but this way you can see how much the telemetry data takes to upload

MS_GOTOOLCHAIN_TELEMETRY_INSTRUMENTATION_KEY will be removed once we have the final instrumentation key, which will be hardcoded.
MS_GOTOOLCHAIN_TELEMETRY_ALLOW_GO_DEVEL will probably stay, but won't be necessary for users outside our team.

I've done some local benchmarking, and the telemetry upload takes ~300ms, which is a reasonable amount of time.

Closes #1672.

Note for reviewers: you don't really need to review most of the new lines. Skip those under src/cmd/vendor, as they are github.com/microsoft/go-infra/telemetry being vendored.