perf(childprocess): spawned processes are tracked and monitored.

[Hweinstock]

Problem

TODO: further investigate flaky test on windows.

Toolkit/Q slow performance is consistently reported by customers: https://github.com/aws/aws-toolkit-vscode/issues?q=is%3Aopen+is%3Aissue+label%3Aperformance

Toolkit/Q features can spawn processes, included many long-lived processes such as LSP servers. The cost of these processes is hidden and hard to troubleshoot. Any misbehaving process will not be noticed except in a coarse "Toolkit/Q causes vscode to be slow" manner.

Solution

• Track all running processes in a map-like data structure.
• use a PollingSet to periodically "monitor" their system usage(cpu%, memory bytes), logging warning messages when it exceeds a threshold.
• Add developer command to instantiate spawned processes via ChildProcess wrapper to make this easier to test.

Future Work

• Integrate telemetry into the tracker instead of only logging.
• Add public-facing (non-dev) command so that users can view active processes and their usages.

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