Use lsprotocol's converter for (de)serialization

This simplifies much of the (de)serialization code by relying on the
converter provided by `lsprotocol`.

We use the `METHOD_TO_TYPES` mapping to determine which type definition
to use for any given message. If a method is not known (as in the case
of custom lsp commands) we fall back to pygls's existing generic RPC
message classes.

The following changes to the base `JsonRPCProtocol` class have also been
made

- server and client futures have been unified into a single
  `_request_futures` dict.
- upon sending a request, the corresponding result type is looked up and
  stored in an internal `_result_types` dict.
- (de)serialization code has been moved to a method on the
  `JsonRPCProtocol` class itself so that it has access to the required
  internal state.
- subclasses (such as the `LanguageServerProtocol` class) are now
  required to implement the `get_message_type` and `get_result_type`
  methods to provide the type definitions corresponding with the given
  RPC method name.

pygls/exceptions.py

@@ -40,11 +40,12 @@ def __hash__(self):
         return hash((self.code, self.message))
 
     @staticmethod
-    def from_dict(error):
+    def from_error(error):
         for exc_class in _EXCEPTIONS:
-            if exc_class.supports_code(error['code']):
-                return exc_class(**error)
-        return JsonRpcException(**error)
+            if exc_class.supports_code(error.code):
+                return exc_class(code=error.code, message=error.message, data=error.data)
+
+        return JsonRpcException(code=error.code, message=error.message, data=error.data)
 
     @classmethod
     def supports_code(cls, code):

pygls/lsp/__init__.py

@@ -36,32 +36,6 @@
     TEXT_DOCUMENT_SEMANTIC_TOKENS_RANGE:  Union[SemanticTokensLegend, SemanticTokensRegistrationOptions],
 }
 
-@attrs.define
-class JsonRPCNotification:
-    """A class that represents json rpc notification message."""
-
-    jsonrpc: str
-    method: str
-    params: Any
-
-@attrs.define
-class JsonRPCRequestMessage:
-    """A class that represents json rpc request message."""
-
-    jsonrpc: str
-    id: Union[int, str]
-    method: str
-    params: Any
-
-
-@attrs.define
-class JsonRPCResponseMessage:
-    """A class that represents json rpc response message."""
-
-    jsonrpc: str
-    id: Union[int, str]
-    result: Union[Any, None] = attrs.field(default=None)
-
 
 def get_method_registration_options_type(
     method_name, lsp_methods_map=METHOD_TO_TYPES