Add serde feature gated support for (de)serialization of packed protos.

[spwilson27]

Add support for serde to enable deserialization inter-op with JSON.

Use Case

I'd like to define a typed config specification using capnp definitions, both to support strong typing and because the config will be sent across IPC boundaries. I'd like to write the actual config file in JSON since it's easily human readable.

Testing

Verified all existing tests pass with cargo nextest run on M4 MAX Mac. Verified example builds and prints expected output:

cargo run
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.26s
     Running `<redacted>/json_config`
{
  "appName": "hive-worker",
  "port": 7000,
  "verbose": true
}

[spwilson27]

Note, code was mostly AI generated. I've manually reviewed it all for anything egregious.

Potential Optimizations:

• We could switch to cfg gating for compilation of functions if that's preferred over runtime checks
• It might be reasonable to split into two separate features, one for serialize, one for deserialize

I'm willing to make changes as requested, sharing this patchset as I've wanted this ability across multiple projects.

[stepancheg]

I think this PR needs some tests. (I am not a maintainer of this project.)