Proof of concept: Allow selecting of slices of assay meta in graphql

[dancoates]

demo query:

query MyQuery {
  project(name: "greek-myth") {
    samples {
      id
      assays {
        id
        type
        coverage: metaValue(path: "$.coverage")
        facility: metaValue(path: "$.facility")
        emoji: metaValue(path: "$.emoji")
      }
    }
  }
}