MCO-1928: Enhacement of the MCO OS streams feature

[pablintino]

This commit adds the formal enhancement proposal for MCO-1914.

This feature adds the ability to run multiple OS base images simultaneously in a cluster by allowing users to select an image 'stream' on a per-MCP basis.

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@pablintino: This pull request references MCO-1928 which is a valid jira issue.

Warning: The referenced jira issue has an invalid target version for the target branch this PR targets: expected the story to target the "4.21.0" version, but no target version was set.

In response to this:

This commit adds the formal enhancement proposal for MCO-1914.

This feature adds the ability to run multiple OS base images simultaneously in a cluster by allowing users to select an image 'stream' on a per-MCP basis.

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[yuqi-zhang]

To answer a question from the API review office hours, the alternatives we considered are:

1. expanding on the existing configmap to refer to more images
2. having this as a status field on an existing object, i.e. the MachineConfiguration cluster object

1 was deemed less optimal since it's an arbitrary map without guardrails, and also with a new API we can use VAP to wire up cross-object validation
2 was deemed less optimal since it's managed by the CVO intead of the MCO, and thus the bootstrap-time MCO doesn't understand or process it (I suppose we could set that up, though). We would like to have a bootstrap (install) time workflow that matches the in-cluster workflow that we can process in the same way, possibly eventually as a install-config field, so the new API object that the MCO manages would be easier to work in both cases.

Also, the MachineConfiguration has introduced quite a lot of new fields in the past few versions, mostly for cluster-knobs (so configurables, instead of status-only discovery objects), so we thought a new CR would be clearer.