scripts: west_commands: Support out-of-tree runners

Add runners entry to the module schema and import the file specified.

Every class that inherits from ZephyrBinaryRunner will be discovered and
added to runners list.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>
diff --git a/scripts/zephyr_module.py b/scripts/zephyr_module.py
index ae2c35c..22369aa 100755
--- a/scripts/zephyr_module.py
+++ b/scripts/zephyr_module.py
@@ -174,6 +174,15 @@
             type: seq
             sequence:
               - type: str
+  runners:
+    required: false
+    type: seq
+    sequence:
+      - type: map
+        mapping:
+          file:
+            required: true
+            type: str
 '''
 
 MODULE_YML_PATH = PurePath('zephyr/module.yml')