Protobuf derive PoC

[Mirko-von-Leipzig]

This PR demonstrates a derive style macro for our protobuf schema. This only implements it for the decoding half, but it can be trivially expanded to support encoding in a similar fashion.

This is implemented in a stand-alone fashion as a completely independent miden-protobuf crate, with a miden-protobuf-derive child crate. The latter is not used directly, the sole entry-points are via the miden-protobuf crate. The intention is that both miden-objects and the node code can use this crate at build, and runtime, to align error formatting and remove the bulk of the (currently) manual implementation.

This PoC demonstrates the usage on the ProtocolConfig type, so the bulk of the "savings" aren't actually visible. There are some minor fluff diffs which are because ConversionError was moved to miden-protobuf.

Note that we also enforce optional markings i.e. the policy is that fields are required unless they are marked by optional in the schema.

The benefits are:

• error handling and mapping are automated i.e. context etc doesn't have to be made manually
• much less hand written code, you just define the constructor mapping
• consistency
• automatic optional handling

[Mirko-von-Leipzig]

This demonstrates the primary API. Callers link the protobuf type to a domain type, and define the constructor ordering and T versus Result<T>.

This injects the proto derive macro which in turn automatically implements an appropriate TryFrom<proto::T> -> Result<domain::T, ConversionError>. We would similarly do this for encoding.