feat(stackable-webhook)!: Add support for mutating webhooks

[sbernauer]

Description

Part of stackabletech/commons-operator#111
Used by stackabletech/commons-operator#387

BREAKING: Refactor the entire WebhookServer mechanism, so multiple webhooks can run in parallel.
Put individual webhooks (currently ConversionWebhook and MutatingWebhook) behind the Webhook trait.

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

Author

• Changes are OpenShift compatible
• CRD changes approved
• CRD documentation for all fields, following the style guide.
• Integration tests passed (for non trivial changes)
• Changes need to be "offline" compatible

Reviewer

• Code contains useful comments
• Code contains useful logging statements
• (Integration-)Test cases added
• Documentation added or updated. Follows the style guide.
• Changelog updated
• Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

• Feature Tracker has been updated
• Proper release label has been added

[NickLarsenNZ]

Left some comments.

Also a few things for me to check into which shouldn't block you.