Proc macros to reduce boiler plate

Dapr seems quite easy to get going in other languages that provide annotations/attributes.
I think the same should be done for the Rust SDK.

For example:

```cs
[Topic("pubsub", "orders")]
[HttpPost("/checkout")]
public async Task>Checkout(Order order, [FromServices] DaprClient daprClient)
{
// Logic
return order;
}
```

Could be expressed as:

```rs
#[topic(pub_sub_name = "pubsub", topic = "orders")]
#[post("/checkout")]
async fn hello(order: Order) -> String {
format!("Order id: {}", order.id)
}
```

Am I right in thinking that the difficulty comes in the variety of HTTP server crates? Or is that quite decoupled from decorating the handler?

Start by surveying how the Rust SDK handles HTTP server crates and whether handler decoration can be decoupled from them. Define the supported proc-macro API and its integration boundaries before implementation; done means the project has an agreed design and working annotations across the intended server integrations.

Written by the indexing model from the issue text.