Support access to MethodOptions fields

[sleipnir]

Is your feature request related to a problem? Please describe.
The option field in methods can be used to access contextual information. an example of this would http transcoding:

service ShoppingCart {
    rpc AddItem(AddLineItem) returns (google.protobuf.Empty) {
        option (google.api.http) = {
            post: "/cart/{user_id}/items/add",
            body: "*",
        };
        option (.cloudstate.eventing).in = "items";
    }

    rpc RemoveItem(RemoveLineItem) returns (google.protobuf.Empty) {
        option (google.api.http).post = "/cart/{user_id}/items/{product_id}/remove";
    }

    rpc GetCart(GetShoppingCart) returns (Cart) {
        option (google.api.http) = {
          get: "/carts/{user_id}",
          additional_bindings: {
            get: "/carts/{user_id}/items",
            response_body: "items"
          }
        };
    }

[polvalente]

@sleipnir do you want to take this on?

[sleipnir]

@sleipnir do you want to take this on?

@polvalente I think we can close this one once the options have been added and that @drowzy is developing #274 via #273