feat: Provide command to reload config in place, fixes #44

[das-peter]

The Issue

• Add command to reload config #44
  For rapid vcl development it would be handy to have a in-place config reloading feature for the varnish configuration.

How This PR Solves The Issue

Provides a command to achieve exactly that.

Manual Testing Instructions

ddev add-on get https://github.com/CandoImage/ddev-varnish/archive/refs/heads/20250804_das_peter_44_provide-config-reload-command.tar.gz
ddev restart
ddev varnish-config-reload

Automated Testing Overview

Added a test for the command since it is a more involved one than the other "pass-through" commands.
The test just has to verify that all the handling from the varnishadm returns the expected outputs to verify a reload is working in "theory".

Release/Deployment Notes

[stasadev]

Thank you!

In addition to my suggestions, please add this new command to the README.md table https://github.com/ddev/ddev-varnish#helper-commands

[das-peter]

@stasadev Thanks for the feedback! All things should be addressed now.

[stasadev]

Looks good to me!