[Feature Request] Allow `Feature`-s specification for Compute nodes

In our use case, it would be extremely useful to allow specifying `Feature`-s for compute nodes in `ComputeResource`. The specified features list should be merged with the features that ParallelCluster currently generates.

Possible generic use case: Cluster admin can specify features for compute nodes and then either create custom partitions with fetaure-based filters or allow users to filter nodes for their jobs directly by feature.

Start by tracing how ComputeResource configuration is handled and where ParallelCluster generates compute-node features. Inspect the existing feature-generation and validation paths, then determine how configured features should merge with generated features and how that behavior should be tested. Done means compute nodes accept the specified features without losing generated ones.

Written by the indexing model from the issue text.