Respect user defined defaults for variables

[olirice]

What kind of change does this PR introduce?

When a user's query defines a default value, that value is currently ignored.

  query Units($first: Int = 3) {
    unitsCollection(first: $first) { 
      edges {
        node {
          id
          name
          desc
        }}}}

This PR changes the behavior so that the user defined default is respected when no value is explicitly passed.

Order of precedence is:

• User provided value in a variable
• User provided default in operation definition
• Server side default value

Tasks

• plumb the variable_definitions through everywhere
• implement the user defined variable logic
• tests, including value order of precedence

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resolves #453