Inadequate forward compatibility in derivations

[roberth]

Describe the bug

Derivations did't have a versioning mechanism; only requiredFeatures can be used for making older Nixes reject derivations they're not capable of building.

RFC 92 dynamic derivations work around this by intentionally breaking the first token (Derivation -> DrvWithVersion).
This created a slot for somewhat of a version string, for which currently the only valid value is "xp-dyn-drv".
While it is a step in the right direction, it is still far from a solution.
Consider also that we may have multiple viable build capability implementations besides the one in this repo.

Steps To Reproduce

Consider the introduction of unsafeDiscardReferences. Nix versions that don't support it, will happily build such derivations, and realise outputs that have references they shouldn't.
This case is particularly bad because no error is raised and outputs are assumed to be correct and immutable by all sorts of other features, so a bad output is not easy to recover from.

Expected behavior

Nix should fail when it's not capable of building a derivation with an unknown feature.

What I currently hold as the best mechanism is a set of feature labels, not unlike requiredFeatures, but for which it is not required to configure them in the remote builder list. We may also come to agreement on "standard" features to establish feature levels that can reasonably be expected to be implemented, reducing the number of features that need to be enabled in any given modern derivation. A feature level would simply be a feature that implies other features, including previous levels.

Additional context

Priorities

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