Deprecate ArgumentValue<T> in favor of FieldValue<T>

[bclozel]

I noticed that our current ArgumentValue<T> support is located in the org.springframework.graphql.data package and focusing on binding data as controller method arguments. But we are already doing more than that:

ArgumentValue is also supported as a field within the object structure of an @Argument method parameter, either initialized via a constructor argument or via a setter, including as a field of an object nested at any level below the top level object.

With #1174 and in general the GraphQL client use case, we should probably generalize this concept as FieldValue<T>, mostly copying the existing code to a new class and using this opportunity to use a more appropriate name and a different package, probably org.springframework.graphql.

If we decide to do so, we would @Deprecate ArgumentValue in favor of FieldValue and remove it entirely in a future version, probably 2.0.

Most of what's described here is already done in a branch, but we'll need to discuss this as a team and test this arrangement with GraphQL clients.

[bclozel]

This has been reverted in favor of exploring alternatives with #569