lib.ExecContext.username: Add property

Add the property .username, backed by the protected _username()
callback. It should return the user run()'s cmd parameter is executed
as.

Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/pkg/lib/ExecContext.py

@@ -240,9 +240,13 @@ class ExecContext(Base):
         super().__init__(uri=uri, interactive=interactive, verbose_default=verbose_default)
 
     @abc.abstractmethod
-    async def _run(self, *args, **kwargs) -> Result:
+    def _username(self) -> str:
         pass
 
+    @property
+    def username(self) -> str:
+        return self._username()
+
     async def run(
         self,
         cmd: list[str],