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/tests/into_command_definition/enum_fails.rs

@@ -0,0 +1,10 @@
+use api_proc_macro::IntoCommandDefinition;
+
+#[derive(IntoCommandDefinition)]
+enum TestEnum {
+    Variant
+}
+
+fn main() {
+
+}

api-proc-macro/tests/into_command_definition/enum_fails.stderr

@@ -0,0 +1,5 @@
+error: IntoCommandDefinition not supported for enum
+ --> tests/into_command_definition/enum_fails.rs:4:1
+  |
+4 | enum TestEnum {
+  | ^^^^

api-proc-macro/tests/into_command_definition/union_fails.rs

@@ -0,0 +1,12 @@
+use api_proc_macro::IntoCommandDefinition;
+
+#[derive(IntoCommandDefinition)]
+#[repr(C)]
+union TestUnion {
+    f1: u32,
+    f2: f32,
+}
+
+fn main() {
+
+}

api-proc-macro/tests/into_command_definition/union_fails.stderr

@@ -0,0 +1,5 @@
+error: IntoCommandDefinition not supported for union
+ --> tests/into_command_definition/union_fails.rs:5:1
+  |
+5 | union TestUnion {
+  | ^^^^^

api-proc-macro/tests/into_command_definition/unnamed_struct_fails.rs

@@ -0,0 +1,8 @@
+use api_proc_macro::IntoCommandDefinition;
+
+#[derive(IntoCommandDefinition)]
+struct TestUnnamedStruct(f32, f64, bool);
+
+fn main() {
+
+}

api-proc-macro/tests/into_command_definition/unnamed_struct_fails.stderr

@@ -0,0 +1,5 @@
+error: IntoCommandDefinition not supported for unnamed structs
+ --> tests/into_command_definition/unnamed_struct_fails.rs:4:25
+  |
+4 | struct TestUnnamedStruct(f32, f64, bool);
+  |                         ^^^^^^^^^^^^^^^^