CPLAT-10037: Add a tool to make managing background processes easier.

[evanweible-wf]

CPLAT-10037

We have a few consumers that start a background process before running a dev tool like tests and then need to stop that process at the end. This PR adds a BackgroundProcessTool to make that easier:

final testServer = BackgroundProcessTool(
    'node', ['tool/server.js'],
    delayAfterStart: Duration(seconds: 1));
final config = {
  'test': CompoundTool()
    ..addTool(testServer.starter, alwaysRun: true)
    ..addTool(TestTool())
    ..addTool(testServer.stopper, alwaysRun: true),
};

[aviary2-wf]

Security Insights

No security relevant content was detected by automated scans.

Action Items

• Review PR for security impact; comment "security review required" if needed or unsure
• Verify aviary.yaml coverage of security relevant code

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[todbachman-wf]

Looks great!

[evanweible-wf]

QA +1

• Tests passing and verified locally with sockjs_client_wrapper

@Workiva/release-management-p

[rmconsole-wf]

+1 from RM