api: add instrospect endpoint

Fixes #1091

doc/api/garage-admin-v2.json

@@ -869,6 +869,40 @@
         }
       }
     },
+    "/v2/IntrospectAdminToken": {
+      "get": {
+        "tags": [
+          "Admin API token"
+        ],
+        "description": "\nReturn information about the calling admin API token.\n    ",
+        "operationId": "IntrospectAdminToken",
+        "parameters": [
+          {
+            "name": "admin_token",
+            "in": "path",
+            "required": true,
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "Information about the admin token",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/IntrospectAdminTokenResponse"
+                }
+              }
+            }
+          },
+          "500": {
+            "description": "Internal server error"
+          }
+        }
+      }
+    },
     "/v2/GetNodeInfo": {
       "get": {
         "tags": [
@@ -2644,6 +2678,54 @@
           }
         }
       },
+      "IntrospectAdminTokenResponse": {
+        "type": "object",
+        "required": [
+          "name",
+          "expired",
+          "scope"
+        ],
+        "properties": {
+          "created": {
+            "type": [
+              "string",
+              "null"
+            ],
+            "format": "date-time",
+            "description": "Creation date"
+          },
+          "expiration": {
+            "type": [
+              "string",
+              "null"
+            ],
+            "format": "date-time",
+            "description": "Expiration time and date, formatted according to RFC 3339"
+          },
+          "expired": {
+            "type": "boolean",
+            "description": "Whether this admin token is expired already"
+          },
+          "id": {
+            "type": [
+              "string",
+              "null"
+            ],
+            "description": "Identifier of the admin token (which is also a prefix of the full bearer token)"
+          },
+          "name": {
+            "type": "string",
+            "description": "Name of the admin API token"
+          },
+          "scope": {
+            "type": "array",
+            "items": {
+              "type": "string"
+            },
+            "description": "Scope of the admin API token, a list of admin endpoint names (such as\n`GetClusterStatus`, etc), or the special value `*` to allow all\nadmin endpoints"
+          }
+        }
+      },
       "ImportKeyRequest": {
         "type": "object",
         "required": [