Enhance/Centralize Configure machinery

[allenss-amazon]

There are many places in the code where there are constant values that should be configurable. Open source has a configuration mechanism for configurable values in module and our code should be using that.

This would be made easy for developers to have a few static classes within VMSDK that you could just statically declare and this
would automatically make that variable configurable. Specific features that should be include:

1. For numerical configurables: Ability to specify range of validity ([min..max]) either as constant values or the result of some invokeable validation function.
2. A programmable validation function. Sometimes a fixed range isn't sufficient, for those cases a callback for code that verifies.
3. Notification on change. A registerable callback to get notification on change.
4. Marking that change is only valid at startup. A special case of verification function, but happens often enough that it's worth providing as a service....

Some examples:

// An integer configurable with default value 10,  valid values in the range [1..100] Can only be set at startup.
VMSDK::Configure<int> x("max-fred-value", 10, 1, 100, VMSDK:ConfigureFlag::kOnlyAtInitialization);

// A float configurable with a default value of 1.5, minimum value is dynamically computed (here by a lambda) max value is 100.
VMSDK::Configure<float> y("frobisher"), 1.5, [] {return ...};);

// An int that invokes a call back when it's changed
VMSDK::Configure<int> z("foobar", 10, [] { .... Called when changed ... });