[server] How to handle "reusable" endpoint/topic definitions?

[jamesmunns]

The current endpoints! and topics! macros don't have any way to "include" definitions that are made externally, for example if you have a common subset of functionality.

We might want to add some way to declare "includes", so you can avoid re-defining types. This could look something like:

endpoints!{
    list = ENDPOINTS_LIST;
    // These are defined somewhere else using the `endpoint!` macro, and are
    // added to the table below as if they were defined here.
    import = [
        shared_icd::AdminEndpoint,
        shared_icd::IdentEndpoint,
    ];
    | EndpointTy     | RequestTy     | ResponseTy    | Path              |
    | ----------     | ---------     | ----------    | ----              |
    | Endpoint1      | Req1          | Resp1         | "endpoints/one"   |
    | Endpoint2      | Req2          | Resp2         | "endpoints/two"   |
}