[Frontend] Add support for Memory Based Scheduling with Multiple Instance Types in ParallelCluster 3.7.0+.

[gmarciani]

Changes

Add support for Memory Based Scheduling with Multiple Instance Types in ParallelCluster 3.7.0+.

Known Limitations
The wizard does not remove the schedulable memory property in the resulting cluster config when mem based scheduling is enabled and then disabled. This is not a blocking bug because the resulting cluster config is valid, but simply an unexpected behaviour that we may decide to address in another iteration.

How Has This Been Tested?

1. Unit tests
2. Manual verification with Pcluster 3.7.0: mem based scheduling can be toggled and preserved with input cluster config when using multiple instance types in 3.7.0. Verified with dryrun creation.
3. Manual verification with Pcluster 3.6.0: feature flagging is working as expected prohibiting the use of mem based scheduling with multiple instance types. Verified with dryrun creation.

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