feat: add ddev redis-backend for swappable backends, fixes #32, fixes #28

[stasadev]

The Issue

• Feature: Swappable Redis-Compatible KV stores  #32
• Valkey as additional service - redis alternative #28

How This PR Solves The Issue

Adds ddev redis-backend command.

TODO: it needs some bats tests and manual tests.

Manual Testing Instructions

ddev add-on get https://github.com/ddev/ddev-redis/tarball/20250424_stasadev_redis_backend
ddev restart

ddev redis-backend

# apply redis:7 optimized config
ddev redis-backend redis optimized
ddev restart

# apply redis:7 default config
ddev redis-backend redis
ddev restart

# apply valkey/valkey:8 default config
ddev redis-backend valkey
ddev restart

Automated Testing Overview

Release/Deployment Notes