[Epic] Fetch policies

[rbalicki2]

• There should be fetch policies: yes no if-necessary etc.
• These are distinct from suspense policies
• Whether an entrypoint is compatible with if-necessary depends on whether the normalization AST is hard required. This should be reflected in the type, and should be controlled by directives.
  • How do we handle multiple entrypoints for the same field, where one entrypoint hard requires the normalization AST and the other doesn't?? Is the required-ness of the normalization AST reflected in the path?? Probably. Perhaps as /entrypoint and /full-entrypoint or something.

[rbalicki2]

• basic check function
• go through missing field handler
• incorporate into useLazyReference etc
• incorporate into loadable fields
• incorporate into expose as and refetch fields (... should refetch fields always default to fetchPolicy: 'Yes'? I think so... but I'm not convinced, because maybe the magic refetch field needs to not exist. Anyway, I will punt on this until we finish Epic: Loadable interface #102)
• check unit tests (see [epic] unit tests for traversals #220 )
• eventually, when normalization ASTs can be lazily loaded, we can do more