feat: add MVP implementation

[martinohmann]

This PR provides an initial implementation for the webhook. The core logic in pkg/admission/handler.go has a test coverage of > 95%.

The general structure of this webhook roughly follows the example from the controller-runtime repository: https://github.com/kubernetes-sigs/controller-runtime/tree/master/examples/builtins

The webhook reads its configuration from a config.yaml file. You can find an example config at config.sample.yaml.

What's not included in this PR and will be added in followups to keep this change reviewable:

• Config validation needs to be added to make the webhook fail to start with a helpful error when the config is invalid
• GitHub Action to build docker images and push them to dockerhub (or ghcr.io, or both 😉 ) when a new tag is pushed
• Helm Chart and helm repo setup via https://github.com/helm/chart-releaser-action
• Documentation for usage and deployment instructions
• Nice-to-have: more configuration options. For example instead of just matching images based on prefixes we could also support regexp matching and the usage of capture groups in the replacement pattern to make this more flexible