Fail fast if managed task workers are near-OOM

[ranweiler]

Summary

• Add onefuzz::memory::available_bytes() to enable checking system-wide memory usage
• In managed task worker runs, heuristically check for imminent OOM conditions and try to exit early

Testing

• Added unit tests for limited parsing of /proc/meminfo on Linux
• Memory querying functionally tested locally on both OSes using memory example binary in onefuzz crate
• Near-OOM check in onefuzz-agent functionally tested by forcing external on-VM OOM
  • Linux
  • Windows

Tested on Windows by using notmyfault from SysInternals to leak from the paged memory pool, on a node where a healthy task was in the running state. On Linux, disabled OOM-killer, set overcommit to always-allow, and ran small custom program that rapidly leaks heap memory. Checked for task failure, with the reason matching the new error message, as well as telemetry.

Closes #1633.