Allow full override of default runtime images

Context

From PR #72 review feedback by @Lytol: the current additionalRuntimeImages field only allows appending to the hardcoded default images. It doesn't let operators override or remove the built-in defaults.

It could be useful to set all images explicitly, falling back to sensible defaults if none are specified.

Current Behavior

• Default runtime images are hardcoded in DefaultRuntimeYAML()
• additionalRuntimeImages appends to those defaults
• No way to remove or replace a hardcoded default image without an operator code change

Proposed Behavior

Allow operators to explicitly define the full set of runtime images, with the current hardcoded list used only as a fallback when no explicit images are specified.

Design Considerations

• Full replace semantics: operators must re-specify every image they want, but new defaults from operator upgrades won't appear automatically
• Additive model (current): safer, but no way to drop stale defaults
• Possible middle ground: a runtimeImages field for full replacement alongside additionalRuntimeImages for appending, or a single field with a mode selector

References

• PR #72: feat: add R 4.5.x default runtime image and configurable additionalRuntimeImages

Start by reading DefaultRuntimeYAML() and the current additionalRuntimeImages handling, then review PR #72 for the existing configuration context. Before implementing, resolve whether full replacement uses a new runtimeImages field or a mode selector. Done means operators can explicitly replace or remove defaults while the current hardcoded list remains the fallback when no explicit images are set.