[Proof of concept] Custom key

[ramiel]

⚠️ This is not a real PR, just a working proof of concept to address some existing issues.
⚠️ The changeset is big because I need to use this PR from github directly. Only visitor.js has relevant changes

• What kind of change does this PR introduce?
  Feature

• What is the current behavior? (You can also link to an open issue here)
  The key cannot be changed dynamically

• What is the new behavior (if this is a feature change)?
  A feature for which is possible to customise the key even if it's autogenerated. This address several issues as Conditional Fetching #157 and Feature Request: Ability to override auto-generated key #158

• Other information:
  The idea is to enable:

Conditional fetching

const result = sdk.useMyQuery(variables, undefined, {
  // This option let you customise the generated key. The key is passed as parameter to the function
  customKey: (key) => isTrue ? key : null
})

override the generated key

const result = sdk.useMyQuery(variables, undefined, {
  // This option let you customise the generated key. The key is passed as parameter to the function
  customKey: (key) => ([...key, customValue])
})