Replace gRPC Backend (#10)

**Rationale:**

Having two separate servers and communication methods resulted in additional maintenance & the need to convert often between backend & frontend data types.
By moving the backend communication off of gRPC and to just use websockets it both gives more control & allows for simplification of the implementation.

#8

**Changes:**

- Replaces gRPC backend.
  - New implementation automatically handles reconnect logic
- Implements an api layer
- Migrates examples to the api layer
- Implements a proc macro to make command handling easier
- Implements unit tests for the api layer (90+% coverage)
- Implements integration tests for the proc macro (90+% coverage)

Reviewed-on: #10
Co-authored-by: Sergey Savelyev <sergeysav.nn@gmail.com>
Co-committed-by: Sergey Savelyev <sergeysav.nn@gmail.com>

api-proc-macro/src/lib.rs

@@ -0,0 +1,11 @@
+extern crate proc_macro;
+
+use proc_macro_error::proc_macro_error;
+
+mod into_command_definition;
+
+#[proc_macro_error]
+#[proc_macro_derive(IntoCommandDefinition)]
+pub fn derive_into_command_definition(item: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    into_command_definition::derive_into_command_definition_impl(item)
+}