Write an RFD for a Rust SDK proc-macro crate

[j-lanson]

We could further simplify writing plugins by having a proc-macro that we can use to tag functions as "query endpoints" and then extract their name, input type / output type to produce a struct implementing Query.

impl Serialize for InputObject { ... }
impl Serialize for OutputObject { ... }

#[this_is_a_sdk_query(default = true)]
async fn query_endpoint(engine: &mut PluginEngine, input: InputObject) -> Result<OutputObject>

-->

1. generate Schemars json objects for InputObject / OutputObject

struct ThisIsAnSdkQuery {}
impl Query for ThisIsAnSdkQuery {
   fn input_schema(&self) -> JsonSchema { return generated Schemars object };
   fn input_schema(&self) -> JsonSchema { return generated Schemars object };
   async fn run(&self, engine: &mut PluginEngine, input: JsonValue) -> Result<JsonValue> {
      let input: InputObject = serde_json::from_value(input)?;
      let res = query_endpoint(engine, input);
      res.map(|v| serde_json::to_value(v))
   }
}

And create a NamedStruct instance for it that we add to some global vec.