maturin: add FeatureSpec for features field

src/schemas/json/maturin.json

@@ -55,10 +55,10 @@
       }
     },
     "features": {
-      "description": "Space or comma separated list of features to activate",
+      "description": "List of features to activate.\nEach entry can be a plain feature name string, or a conditional object\nwith `feature` and `python-version` keys.",
       "type": ["array", "null"],
       "items": {
-        "type": "string"
+        "$ref": "#/definitions/FeatureSpec"
       }
     },
     "frozen": {
@@ -231,6 +231,30 @@
         }
       }
     },
+    "FeatureSpec": {
+      "description": "A cargo feature specification that can be either a plain feature name\nor a conditional feature that is only enabled for certain Python versions.\n\n# Examples\n\n```toml\n[tool.maturin]\nfeatures = [\n  \"some-feature\",\n  { feature = \"pyo3/abi3-py311\", python-version = \">=3.11\" },\n  { feature = \"pyo3/abi3-py38\", python-version = \"<3.11\" },\n]\n```",
+      "anyOf": [
+        {
+          "description": "A plain feature name, always enabled",
+          "type": "string"
+        },
+        {
+          "description": "A feature enabled only when the target Python version matches",
+          "type": "object",
+          "properties": {
+            "feature": {
+              "description": "The cargo feature to enable",
+              "type": "string"
+            },
+            "python-version": {
+              "description": "PEP 440 version specifier for the target Python version, e.g. \">=3.11\"",
+              "type": "string"
+            }
+          },
+          "required": ["feature", "python-version"]
+        }
+      ]
+    },
     "Format": {
       "description": "The target format for the include or exclude [GlobPattern].\n\nSee [Formats].",
       "oneOf": [