Add plugin architecture

example/plugin/decman_plugin_example.py

@@ -0,0 +1,10 @@
+import os
+
+import decman
+
+
+class Example(decman.Plugin):
+    NAME = "example"
+
+    def available(self) -> bool:
+        return os.path.exists("/tmp/example_plugin_available")

example/plugin/pyproject.toml

@@ -0,0 +1,14 @@
+[project]
+name = "decman-plugin-example"
+version = "0.1.0"
+requires-python = ">=3.13"
+dependencies = [
+    "decman",
+]
+
+[project.entry-points."decman.plugins"]
+example = "decman_plugin_example:Example"
+
+[build-system]
+requires = ["setuptools>=61.0"]
+build-backend = "setuptools.build_meta"