Create placeholder variable values, even when no "provider" function is defined

[ErikWittern]

As it stands, this library relies on provider functions / values to produce values for variables used in a generated query.

To make the library easier to use out of the box, the library could instead produce placeholder values, e.g.:

• A small number like 10 or 10.0 for variables of type Int or Float
• A string like "PLACEHOLDER" for variables of type String or ID
• true for variables of type Boolean
• Mock input objects whose leaf fields are filled with scalars as described above