t3c makes too many environment-level assumptions w/o any ability to override

[mcclory]

Current behavior:

• When running t3c apply on a given instance running trafficserver, it makes a bunch of assumptions about pathing
  • Setting the TS_HOME env var does allow for setting the base pathing for configuration
  • There is no way to set the plugin path that's referenced in the t3c-check-refs - -p/--trafficserver-plugin-dir - https://github.com/apache/trafficcontrol/blob/2eb00e0cfd8e83ee77ea6016ceaaa272538d55a3/cache-config/t3c-check-refs/README.md
• if you use the OS-level install packages, at least on rocky 8, the plugin path is not at the pre-configured/default path of /opt/trafficserver/libexec/trafficserver and there doesn't appear to be a meaningful way to pass a value that would get passed to the t3c-check-refs calls that happen within t3c apply

Impact:

• It's not possible to use t3c properly on a cache host (edge/mid) that's been installed with OS-level packages

New behavior:

• Proposal would be to leverage environment variable similar to TS_HOME to set the default of this value to make it possible to pass an environment-specific/custom path where trafficserver plugins can be found
  • simplest solution would be to introduce a TS_PLUGIN_ROOT env var check which would simply read the env var in and default to the current value (/opt/trafficserver/libexec/trafficserver)
  • while not super ideal, it would at least allow for cache tier servers to use os package installations with some basic env-level overrides in order to allow for t3c to appropriately configure and validate the plugins.config file
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[ocket8888]

Related: #6205