Add JMT proof spec for convenience

[damiannolan]

Was brought to attention by @cmwaters that it would be useful to have JMTSpec added here for convenience.

Right now its a little bit fragmented. Definition in rust is in the penumbra codebase (ref)

And there is a golang definition which I found here in cosmos/relayer (ref)

Proposing to add the JMTSpec definition for both rust and golang to this repository.