abstract value protocol / `ShapeDtypeStruct` distinction

[froystig]

Several of our APIs (jax.linearize, AOT lowering, jax.make_jaxpr, ...) emphasize duck-typed arguments that correspond to abstract values/arrays. We stress in documentation that any object with shape and dtype (etc.) is acceptable. But then we annotate these APIs with the concrete jax.ShapeDtypeStruct, contradicting the docs.

One way forward may be to define a Protocol type, use it as the annotation, and continue to expose jax.ShapeDtypeStruct for convenience.

(It's a bit unclear to me what to name this protocol. ShapeDtypeStruct itself is already arguably a misnomer, since there are other optional fields there too.)