Add the option to include custom VCL through parameters

[AbdelrahmanElawady]

This PR adds the option to include custom VCL through parameters in Varnish cache. This allows importing plugins (VMODs) and using them throughout the VCL file. It allows adding custom logic that is not handled by t3c. For example adding the functionality of some ATS plugins like Header Rewrite and Regex Remap.

It expects the subroutine that the code should be appended to so the user can have custom logic in different parts of the transaction lifecycle.

---

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?

• Using CDN-in-a-Box, edit the edge parameters to include custom VCL. For example here is some code to remove via header in case of a test endpoint:

{
			"configFile": "default.vcl",
			"name": "vcl_deliver",
			"secure": false,
			"value": "if (req.url ~ \"/test\") {unset resp.http.via;}"
}

• Run docker-compose exec enroller curl -L -o /dev/null -D - -s "http://video.demo1.mycdn.ciab.test/test" to print response headers.
• via header should not be printed

PR submission checklist

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

[codecov]

Codecov Report

Merging #7850 (224ce75) into master (23e9248) will decrease coverage by 2.90%.
Report is 35 commits behind head on master.
The diff coverage is 20.00%.

@@             Coverage Diff              @@
##             master    #7850      +/-   ##
============================================
- Coverage     31.87%   28.97%   -2.90%     
  Complexity       98       98              
============================================
  Files           717      603     -114     
  Lines         82756    77425    -5331     
  Branches        970       90     -880     
============================================
- Hits          26377    22437    -3940     
+ Misses        54218    52894    -1324     
+ Partials       2161     2094      -67     

Flag	Coverage Δ
golib_unit	53.88% <33.33%> (+0.30%)	⬆️
grove_unit	12.02% <ø> (ø)
t3c_unit	5.80% <0.00%> (-0.12%)	⬇️
traffic_monitor_unit	26.44% <ø> (ø)
traffic_ops_unit	21.68% <ø> (ø)
traffic_portal_v2	?
traffic_stats_unit	10.78% <ø> (ø)
unit_tests	25.81% <20.00%> (-3.39%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
cache-config/t3c-apply/config/config.go	0.00% <ø> (ø)
cache-config/t3c-apply/torequest/cmd.go	0.00% <ø> (ø)
cache-config/t3c-apply/torequest/torequest.go	5.65% <ø> (ø)
cache-config/t3c-apply/util/gitutil.go	0.00% <ø> (ø)
cache-config/t3c-apply/util/util.go	0.00% <ø> (ø)
cache-config/t3c-check-refs/config/config.go	0.00% <ø> (ø)
cache-config/t3c-check-refs/t3c-check-refs.go	1.78% <ø> (ø)
cache-config/t3c-generate/cfgfile/all.go	51.38% <ø> (ø)
cache-config/t3c-generate/cfgfile/routing.go	84.00% <ø> (ø)
cache-config/t3c-generate/cfgfile/sslkeys.go	0.00% <ø> (ø)
... and 64 more

... and 152 files with indirect coverage changes

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