Allow disabling grpc at the build level

[benthecarman]

Not going to use this feature and can't get it to compile the proto files. Makes sense to have this as a build flag imo

[scsibug]

Happy to apply, getting a build error right now though:

❯ cargo build
   Compiling nostr-rs-relay v0.8.13 (/Users/scsibug/repos/nostr-rs-relay)
error[E0308]: mismatched types
   --> src/db.rs:146:32
    |
146 |             grpc_client = Some(nauthz::EventAuthzService::connect(&svr).await)
    |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `Grpc`, found `EventAuthzService`

error[E0599]: no method named `admit_event` found for mutable reference `&mut config::Grpc` in the current scope
   --> src/db.rs:364:22
    |
363 |                   let decision_res = c
    |  ____________________________________-
364 | |                     .admit_event(
    | |                     -^^^^^^^^^^^ method not found in `&mut Grpc`
    | |_____________________|
    | 

Some errors have detailed explanations: E0308, E0599.
For more information about an error, try `rustc --explain E0308`.
error: could not compile `nostr-rs-relay` (lib) due to 2 previous errors