DO NOT MERGE: (WIP) Adds support for Link preprocessing

[nyteshade]

For users of the @apollo/client that use client side schemas, this PR allows the users to provide an ApolloLink for preprocessing that allows configurable modification to the query/mutation DocumentNode before the client performs its own caching and execution.

An example of modifying the DocumentNode to swap a @Persistent directive for a @client directive when the application is offline, or removing it entirely if it online.

RFC: #10228

const client = new ApolloClient({
  cache,
  uri: 'http://someserver.com/graphql',
  preprocessorLink: new ApolloLink((operation) => {
    let newOpSDL = parse(print(operation.query))

    newOpSDL = swapDirective(
      newOpSDL,
      'persistent',
      () => online() ? undefined : 'client'
    )

    return { data: newOpSDL }
  })
})

client.query({ query: gql`query { your { heart { away }}}`})
client.mutate({ mutation: gql`mutation { or { change { things }}}`})

 function swapDirective(document, searchFor, replaceWith) {
  let alteredDocument = document

  const isFn = (o) => /Function/.test(Object.prototype.toString.call(o))
  const target = isFn(searchFor) ? searchFor() : String(searchFor)
  const newValue = isFn(replaceWith) ? replaceWith() : replaceWith && String(replaceWith) | undefined

  const getter = (n) => n?.name?.value || null
  const setter = (n, v) => { if (n?.name) n.name.value = v; return n }
  const hasTarget = (n) => new RegExp(target, 'i').test(getter(n))

  alteredDocument = visit(document, {
    Field: {
      enter(node, key, parent, path, ancestors) {
        if (node.directives?.length && node.directives.some(hasTarget)) {
          if (!!!newValue) {
            for (let i = 0; i < node.directives.length; i++) {
              if (new RegExp(target, 'i').test(getter(node.directives[i]))) {
                node.directives.splice(i, 1)
                i--
              }
            }
          }
          else {
            // Thought process here is to swap @persistent for @client
            // when we are offline.
            node.directives = node.directives
              .filter(directive => new RegExp(target, 'i').test(getter(directive)))
              .map(directive => {
                return setter(directive, newValue)
              })
          }
          return node
        }
      },
    }
  })

  return alteredDocument || document
}

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[apollo-cla]

@nyteshade: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[nyteshade]

What I don't understand is why this await within QueryManager.mutate is causing so many tests to fail. Logic says it shouldn't otherwise cause such issues.

[jerelmiller]

Per this comment, we plan to do this work as part of #10303. Thanks for the discussion and helping us think through a solution!