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/src/lib.rs

@@ -0,0 +1,15 @@
+pub mod client;
+pub mod data_type {
+    pub use api_core::data_type::*;
+}
+pub mod data_value {
+    pub use api_core::data_value::*;
+}
+pub mod messages;
+
+pub mod macros {
+    pub use api_proc_macro::IntoCommandDefinition;
+}
+
+#[cfg(test)]
+pub mod test;