add validate for NodeResourcesAllocatable and Coscheduling plugin

[googs1025]

What type of PR is this?

/kind feature
/kind cleanup

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #887

Special notes for your reviewer:

Does this PR introduce a user-facing change?

None

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[googs1025]

/assign @Huang-Wei
/PTAL thanks

[ffromani]

overall seems OK, some inline comments

[ffromani]

this should probably be done in init or similar initialization function

[googs1025]

done

[ffromani]

if ModeType is an enum-like type (is it right?) how can it be different than the enumerated value? isn't the golang type system preventing this?

[googs1025]

IIUC, we do not have enum restrictions on the configuration file, users may fill in any policy. 🤔

// NodeResourcesAllocatableArgs holds arguments used to configure NodeResourcesAllocatable plugin.
type NodeResourcesAllocatableArgs struct {
	metav1.TypeMeta `json:",inline"`

	// Resources to be considered when scoring.
	// Allowed weights start from 1.
	// An example resource set might include "cpu" (millicores) and "memory" (bytes)
	// with weights of 1<<20 and 1 respectfully. That would mean 1 MiB has equivalent
	// weight as 1 millicore.
	Resources []schedconfig.ResourceSpec `json:"resources,omitempty"`

	// Whether to prioritize nodes with least or most allocatable resources.
	Mode ModeType `json:"mode,omitempty"`
}

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