Allow Feature specification in compute resources

[efim-a-efim]

Description of changes

This change adds support for Feature specification for ComputeResource

Tests

Manual test:

1. Create a cluster config where Feature parameter is specified in Scheduling.SlurmQueues[].ComputeResources[].CustomSlurmSettings
2. Try to run pcluster command(s) to validate it - should go through.

References

This PR is complimentary to a PR in cookbooks repo: aws/aws-parallelcluster-cookbook#3020 - please merge that PR first!

Closes #7039

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[gmarciani]

Thanks for contributing to ParallelCluster!
Really appreciated!

[gmarciani]

[Testing] This change is missing coprresponding changes in unit tests and integration tests.
The reason for missing unit test is that CustomSlurmSettingsValidator takes as input the deny list and we unit test here that a generic deny list given as input is honored.

To improve the testing we need to extend the unit test here to verify that the validator is called with the expected deny list.

The feature captured by this PR and aws/aws-parallelcluster-cookbook#3020, which is its counterpart in the cookbook, requires the extension of the existing end to end test here

We will evaluate internally if we can consider the integration tests out of scope for this PR and be us taking care of it. Will get back to this by tomorrow eod of the day.

Also, have you manually tested this PR together with with aws/aws-parallelcluster-cookbook#3020, which is its required counterpart in the cookbook?

[gmarciani]

[Documentation] this change requires an entry in our changelog. Can you please add

3.15.0
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**ENHANCEMENTS**
- Allow specification of `Features` as custom Slurm setting for compute resources.