[5.x] Support Sections in form GraphQL queries

[ryanmitchell]

This PR adds support for sections in GraphQL queries:

query MyQuery {
  form(handle: "test") {
    title
    rules
    honeypot
    handle
    sections {
      instructions
      display
      fields {
         instructions
         type
         display
         handle
         config
         width
      }
    }
  }
}

Closes #8482

[jasonvarga]

Would you mind adding a test to tests/GraphQL/FormTest?

[ryanmitchell]

Thanks for pointing me in the right direction - I did search for tests I promise! Not sure how I missed them.