[FEATURE] [RPC] Create Procedural Macro for Handler Definitions

[hulxv]

Description

Implement the #[rpc_handler] procedural macro to make handler definition ergonomic.

Motivation

Allow users to write handlers as simple async functions without boilerplate.

Proposed Solution

#[rpc_handler]
async fn add(params: AddParams) -> Result<i32> {
    Ok(params.a + params.b)
}

Tasks

• Create mill-rpc-macros crate
• Implement #[rpc_handler] macro
• Support async functions
• Add parameter extraction
• Handle errors properly
• Support compute pool attribute: #[rpc_handler(compute)]
• Write macro tests
• Document macro usage

Expected

• Ergonomic handler definition
• Automatic parameter extraction
• Error handling integration
• Compute pool support

Dependencies:

• Depends on [FEATURE] [RPC] Implement Router and Handler Trait #40