Add Varnish Support Blueprint (#7620)

* Add Varnish Support Blueprint

* Change package name and remove logrotate

* Remove new t3c subcommands and change cache profile type

blueprints/varnish-support.md

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+# Add Varnish Cache Support
+
+## Problem Description
+
+Currently Traffic Control uses Traffic Server as the underlying cache server. We can expand that by introducing Varnish cache as an option for the cache server used with its great performance, robustness and modularity.
+
+## Proposed Change
+
+From a high level point of view, ATS operates based on configuration files that describe in details how it should work and interact with other servers in the cache hierarchy. These configuration files are managed and generated using `t3c` components that utilize Traffic Ops APIs to get profiles and parameters data required for the configuration files. The proposed change is to use the same data fetched from Traffic Ops APIs to generate configuration files for Varnish cache with almost the same functionality.
+
+Note: the changes should not affect existing components but rather build on them.
+
+### Traffic Portal Impact
+
+n/a
+
+### Traffic Ops Impact
+
+n/a
+
+#### REST API Impact
+
+n/a
+
+#### Client Impact
+
+n/a
+
+#### Data Model / Database Impact
+
+- The profile type `ATS_PROFILE` will be renamed to `CACHE_PROFILE` to indicate that the profile is used for any cache server not just ATS while parameters and other fields won't be affected.
+- `DeliveryService` structs contain fields related to ATS like `remapText`. It will be parsed and translated to Varnish configuration.
+
+### ORT Impact
+
+- `varnishcfg` package will be developed to handle generating configuration files for Varnish, Hitch and `varnishncsa`. For detailed description of mapping configuration files from ATS to Varnish refer to [Varnish Support](https://github.com/apache/trafficcontrol/wiki/Varnish-Support) wiki.
+- New options will be added to `t3c-generate` and `t3c-apply` including `--cache` to indicate which cache server the configuration files will be generated or applied to (e.g. `--cache=varnish` or `--cache=ats`). Flags will be rewritten to indicate which cache server they can be used with. `t3c` subcommands will decide based on `cache` option whether to use `go-atscfg` or `varnishcfg` for configuration files generation and also how to apply them for each case.
+- `go-atscfg` will be refactored to export some of its functionality to be reusable from `varnishcfg`. So, instead of rewriting the logic of which IPs are allowed for specific HTTP requests, it could be separated and exported in a function that both packages utilize.
+
+### Traffic Monitor Impact
+
+New statistics parser will be added to Traffic Monitor to handle data coming from Varnish cache statistics endpoint. There is no `VMOD` that exposes Varnish statistics so a service that keeps polling `varnishstat` will be developed.
+
+### Traffic Router Impact
+
+n/a
+
+### Traffic Stats Impact
+
+n/a
+
+### Traffic Vault Impact
+
+n/a
+
+### Documentation Impact
+
+New documentation will be needed for how to setup Varnish with TC and what is the differences between Varnish and ATS.
+
+### Testing Impact
+
+In addition to unit tests and integration tests, [`varnishtest`](https://varnish-cache.org/docs/trunk/reference/varnishtest.html) could be used to test Varnish cache is operating as expected.
+
+### Performance Impact
+
+For current components there should be no performance impact. However, between Traffic Server and Varnish it isn't clear yet what the difference in performance will be.
+
+### Security Impact
+
+n/a
+
+### Upgrade Impact
+
+n/a
+
+### Operations Impact
+
+n/a
+
+### Developer Impact
+
+n/a
+
+## Alternatives
+
+n/a
+
+## Dependencies
+
+- Varnish and its utilities (`varnishtest`, `varnishstat`, `varnishncsa`, ...).
+- Hitch to handle incoming HTTPS requests as Varnish doesn't support HTTPS.
+
+## References
+
+- https://github.com/apache/trafficcontrol/wiki/Varnish-Support
+- https://varnish-cache.org/docs/trunk/reference/
+- https://varnish-cache.org/vmods/
+- https://github.com/apache/trafficcontrol/wiki/Varnish-Support