Add "FieldResolverStrategies" utility/factory class

[sleberknight]

TestHelpers contains a bunch of useful methods to easily create FieldResolverStrategy instances for a single resolution, e.g., for a specific value, or using a Supplier. Recently I've wanted a few of these in production code. So, create a new utility/factory class that contains methods to easily create instances when you only want a single strategy.

It could be named FieldResolverStrategies and provide all or only a few of methods that exist in TestHelpers currently. Here are the signatures from TestHelpers:

<T> FieldResolverStrategy<T> newSystemPropertyFieldResolverStrategy(String systemPropertyKey)

<T> FieldResolverStrategy<T> newEnvVarFieldResolverStrategy(String envVariable)

<T> FieldResolverStrategy<T> newExternalPropertyFieldResolverStrategy(String externalProperty)

<T> FieldResolverStrategy<T> newExplicitValueFieldResolverStrategy(T explicitValue)

<T> FieldResolverStrategy<T> newSupplierFieldResolverStrategy(Supplier<T> supplier)

The only one I've needed thus far in production code is for an explicit value, so maybe we start with only that method and add others if needed. We should check if one or more of these is already used in existing ConfigProvider implementations, and those are good candidates to include as well.