This introduces the main precheck logic. This will follow concurrency model, timeout handling, and retry logic established in the SPEC. We will follow the decision flow in the [connectivity pre-checks SPEC](https://wiki.cfdata.org/spaces/TUN/pages/1374967685/Connectivity+Pre-checks+for+cloudflared). You can find an attached image of what the decision flow should look like.
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Also update golang.org/x/net and google.golang.org/grpc to fix vulnerabilities,
although cloudflared is using them in a way that is not exposed to those risks
h2mux is already deprecated and will be eventually removed, in the meantime,
the compression tests cause flaky failures. Removing them and the brotli
code slims down our binaries and dependencies on CGO.
Miguel da Costa Martins Marcelino