Remove MergedConfig

[nightkr]

Description

Fixes #456.

This is intended to solve two things:

1. Having to add every new config field to umpteen places, adding a lot of busywork to every change (and a lot of annoying indirection when trying to chase down the real definition of everything).
2. MergedConfig containing the union of all roles' options, making it unclear which fields available when (and the especially comical case where resources exists in all variants... but still has a separate accessor method for each).

Instead, this PR delegates actually-common options to the new struct CommonNodeConfig and introduces a new enum AnyNodeConfig which users can explicitly downcast from to access role-specific options. Common options are accessible immediately because AnyNodeConfig derefs to CommonNodeConfig.

This PR replaces all uses of &dyn MergedConfig with &AnyNodeConfig to minimize impact. In the future, it's intended that new code that is only relevant for a single role should use that role's config type directly.

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
- [x] Changes are OpenShift compatible
- [x] CRD changes approved
- [x] CRD documentation for all fields, following the [style guide](https://docs.stackable.tech/home/nightly/contributor/style-guide).
- [ ] Helm chart can be installed and deployed operator works
- [ ] Integration tests passed (for non trivial changes)
- [ ] Changes need to be "offline" compatible

# Reviewer
- [x] Code contains useful comments
- [x] (Integration-)Test cases added
- [x] Documentation added or updated. Follows the [style guide](https://docs.stackable.tech/home/nightly/contributor/style-guide).
- [ ] Changelog updated
- [x] 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

[razvan]

nice. maybe an entry in the change log would be nice, even if it's not user facing.

[nightkr]

I would argue that these externally invisible changes don't make sense to add, "refactored MergedConfig" isn't something that tells a user.. anything.

[razvan]

This is probably a discipline vs. common sense discussion. I can get behind both arguments, just mentioned it because it's a checkbox for the reviewer.