feat: add helper commands

[tyler36]

This PR adds helper commands for interacting with Varnish.

All commands are designed to:

• run in the Varnish container
• pass through additional parameters from original command.

Commands were sourced from https://varnish-cache.org/docs/6.5/reference/index.html .

[rfay]

Please also add info about these commands to the README.md, thanks!

[tyler36]

• Documentation was added for main readme
• Updated the "Usage" section of each commands

[rfay]

@jedubois what do you think of this? Are you getting notifications from this repo?

[jedubois]

@rfay , looks like a useful addition. @tyler36 , any reason not to include varnishadm?

[jedubois]

@jedubois what do you think of this? Are you getting notifications from this repo?

@rfay , getting notifications but not getting around to following up. Merged this but want to hold off on another release to see about adding varnishadm, think possibly another command some coworkers often use (a simplified cache clear for non-Varnish experts), and look at updating doc reference to 6.0LTS which is base version I think this add-on should stick to.

[rfay]

Thanks @jedubois totally understand you have other things to do, and very much appreciate you maintaining this. It makes so much sense for a real user to maintain things.

[rfay]

BTW, this will be moving to github.com/ddev/ddev-varnish shortly, shouldn't impact anything at all.

[tyler36]

any reason not to include varnishadm?

@jedubois no reason, think I just missed when adding the others.