BackReference should allow pointing to specific array elements

Today James support JMAP BackReferences to allow compose JMAP method calls together within a single JMAP request in order to reduce roundtrips, a la graphQL.

https://jmap.io/spec-core.html#references-to-previous-method-results specify the mechanism: a JSON pointer allow extracting the JSON value to supply from a previously executed method response. It clearly state that https://datatracker.ietf.org/doc/html/rfc6901 should be respected. RFC-6901 allow pointing toward dedicated array items:

Given

   {
      "foo": ["bar", "baz"],
      ...
   }

   The following JSON strings evaluate to the accompanying values:
    
    ...
    "/foo/0"       "bar"
    ...

However this is not currently supported by James.

Confusion arrise from the fact that JMAP relies on JSON pointers for updates too and forbids to do partial updates. However no such limitation is stated in the context of back references.

Thus, in a spec compliance effort, we shall allow pointing back-references to items in arrays.