add t3c-tail sub-app

[jpappa200]

Adds the sub-app t3c-tail which can be used to tail a file, usually a log file. Reads json input from stdin. will mainly be used by t3c-apply to watch diags.log after trafficserver restart or reload. it can be useful to see what happened in diags.log without having to tail the file in a separate window. this will also be logged with the t3c log output.

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Which Traffic Control components are affected by this PR?

• Traffic Control Cache Config (t3c, formerly ORT)

What is the best way to verify this PR?

Build Traffic Control Cache Config and install. run t3c-apply in syncds or badass mode. verify that after reload/restart diags logs messages are displayed.

If this is a bugfix, which Traffic Control versions contained the bug?

PR submission checklist

• [ ] This PR has tests
• [x] This PR has documentation
• [x] This PR has a CHANGELOG.md entry
• [x] This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)