[lldb] [llvm] [lldb-dap] Add unknown request handler (PR #181109)

[Ebuka Ezike via llvm-commits]

================
@@ -1677,6 +1677,14 @@ def request_testGetTargetBreakpoints(self):
         }
         return self._send_recv(command_dict)
 
+    def request_unknown(self):
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da-viper wrote:

```suggestion
    def request_custom(self, command: str, arguments: Optional[dict[str, Any]] = None):
```
Could we change this to `request_custom` as `request_unknown` heavily implies that that is a request that is called unknown. 

https://github.com/llvm/llvm-project/pull/181109