feat: Add method to unregister devices from BlueskyContext (mx-bluesky#987) (#1048)

This PR adds a method `unregister_all_devices()` to `BlueskyContext`

### Review Notes

This functionality is required in the implementation of
* DiamondLightSource/mx-bluesky#987
this PR also raised along with
* DiamondLightSource/dodal#1271

See associated mx-bluesky PR #TBD

---------

Co-authored-by: Joseph Ware <53935796+DiamondJoseph@users.noreply.github.com>

Author: rtuck99

src/blueapi/core/context.py

@@ -232,6 +232,10 @@ def register_device(self, device: Device, name: str | None = None) -> None:
 
         self.devices[name] = device
 
+    def unregister_all_devices(self):
+        """Unregister all devices from the context."""
+        self.devices.clear()
+
     def _reference(self, target: type) -> type:
         """
         Create an intermediate reference type for the required ``target`` type that

tests/unit_tests/core/test_context.py

@@ -385,6 +385,13 @@ def test_device_reference_cache_with_generics(empty_context: BlueskyContext) ->
     assert empty_context._reference(motor) is not empty_context._reference(Movable)
 
 
+def test_unregister_all_devices(devicey_context: BlueskyContext) -> None:
+    assert len(devicey_context.devices) > 0
+    devicey_context.unregister_all_devices()
+
+    assert len(devicey_context.devices) == 0
+
+
 def test_reference_type_conversion(empty_context: BlueskyContext) -> None:
     movable_ref: type = empty_context._reference(Movable)
     assert empty_context._convert_type(Movable) == movable_ref