Support wrapper variables & caching

[mattkrick]

The new wrapper variable is a string & if you want to inject variables into it, you just do it the JS way with backticks.
A better way would be to accept wrapper, wrapperVars.
Then, we could cache the wrapper AST & reuse that a thousand times instead of always having to parse & print.

AC

• githubRequest accepts wrapperVars
• if wrapperVars is present, then variableValue definitions are created for each variable, using the same name.
• a check for duplicate variable names is performed & the names are changed (e.g. team, team1, team2, etc.) until a unique name is found

e.g.

Here's how we do it today

repositories(name: "nest-graphql-endpoint", owner: "parabolinc") {
  id
}

becomes

query($name: String!, $owner: String!) {
  repositories(name: $name, owner: $owner) {
    id
  }
}

+

{name, owner}