[Spike] Investigate using YAML anchors in `bakery.yaml`

This is a possible solution for the issues @bschwedler pointed out with reusing OS and variant definitions in bakery.yaml. I know that ruamel.yaml has support for YAML anchors which could make using common patterns in the config easier and more clear than trying to implement the same global/granular override/extend patterns we've applied to things such as registries.

Probably indent this under images: variants:

Can variants be specified at the top-level of the bakery.yaml file? If not, is this something we might want to add in the future?

Originally posted by @bschwedler in https://github.com/posit-dev/images-shared/pull/159#discussion_r2303933872

Thinking through our discussion of having an OS lookup.

In the future, it may be useful to make an OS list a top-level item in the bakery.yaml file, then this could be simplified to take either a string, and if it's a string, look up the definition from elsewhere.

This would be similar in concept to having variants at the top level.

Originally posted by @bschwedler in https://github.com/posit-dev/images-shared/pull/159#discussion_r2303953497

Read the existing bakery.yaml structure and the linked pull request discussion first. Investigate ruamel.yaml anchor support and whether variants or OS definitions can be placed at the top level, then record the supported approach and its intended configuration shape as the outcome.