Add first draft of admin api

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+# Access control
+
+The admin API uses two different tokens for acces control, that are specified in the config file's `[admin]` section:
+
+- `metrics_token`: the token for accessing the Metrics endpoint (if this token is not set in the config file, the Metrics endpoint can be accessed without access control);
+- `admin_token`: the token for accessing all of the other administration endpoints (if this token is not set in the config file, these endpoints can be accessed without access control).
+
+# Administration API endpoints
+
+## Metrics `GET /metrics`
+
+Returns internal Garage metrics in Prometheus format.
+
+## GetClusterStatus `GET /status`
+
+Returns the cluster's current status in JSON, including:
+
+- Live nodes
+- Currently configured cluster layout
+- Staged changes to the cluster layout
+
+Example response body:
+
+```json
+{
+  "knownNodes": {
+    "ec79480e0ce52ae26fd00c9da684e4fa56658d9c64cdcecb094e936de0bfe71f": {
+      "addr": "10.0.0.11:3901",
+      "is_up": true,
+      "last_seen_secs_ago": 9,
+      "hostname": "node1"
+    },
+    "4a6ae5a1d0d33bf895f5bb4f0a418b7dc94c47c0dd2eb108d1158f3c8f60b0ff": {
+      "addr": "10.0.0.12:3901",
+      "is_up": true,
+      "last_seen_secs_ago": 1,
+      "hostname": "node2"
+    },
+    "23ffd0cdd375ebff573b20cc5cef38996b51c1a7d6dbcf2c6e619876e507cf27": {
+      "addr": "10.0.0.21:3901",
+      "is_up": true,
+      "last_seen_secs_ago": 7,
+      "hostname": "node3"
+    },
+    "e2ee7984ee65b260682086ec70026165903c86e601a4a5a501c1900afe28d84b": {
+      "addr": "10.0.0.22:3901",
+      "is_up": true,
+      "last_seen_secs_ago": 1,
+      "hostname": "node4"
+    }
+  },
+  "layout": {
+    "roles": {
+      "ec79480e0ce52ae26fd00c9da684e4fa56658d9c64cdcecb094e936de0bfe71f": {
+        "zone": "dc1",
+        "capacity": 4,
+        "tags": [
+          "node1"
+        ]
+      },
+      "4a6ae5a1d0d33bf895f5bb4f0a418b7dc94c47c0dd2eb108d1158f3c8f60b0ff": {
+        "zone": "dc1",
+        "capacity": 6,
+        "tags": [
+          "node2"
+        ]
+      },
+      "23ffd0cdd375ebff573b20cc5cef38996b51c1a7d6dbcf2c6e619876e507cf27": {
+        "zone": "dc2",
+        "capacity": 10,
+        "tags": [
+          "node3"
+        ]
+      }
+    },
+    "stagedRoleChanges": {
+      "e2ee7984ee65b260682086ec70026165903c86e601a4a5a501c1900afe28d84b": {
+        "zone": "dc2",
+        "capacity": 5,
+        "tags": [
+          "node4"
+        ]
+      }
+    }
+  }
+}
+```
+
+## GetClusterLayout `GET /layout`
+
+Returns the cluster's current layout in JSON, including:
+
+- Currently configured cluster layout
+- Staged changes to the cluster layout
+
+(the info returned by this endpoint is a subset of the info returned by GetClusterStatus)
+
+Example response body:
+
+```json
+{
+  "roles": {
+    "ec79480e0ce52ae26fd00c9da684e4fa56658d9c64cdcecb094e936de0bfe71f": {
+      "zone": "dc1",
+      "capacity": 4,
+      "tags": [
+        "node1"
+      ]
+    },
+    "4a6ae5a1d0d33bf895f5bb4f0a418b7dc94c47c0dd2eb108d1158f3c8f60b0ff": {
+      "zone": "dc1",
+      "capacity": 6,
+      "tags": [
+        "node2"
+      ]
+    },
+    "23ffd0cdd375ebff573b20cc5cef38996b51c1a7d6dbcf2c6e619876e507cf27": {
+      "zone": "dc2",
+      "capacity": 10,
+      "tags": [
+        "node3"
+      ]
+    }
+  },
+  "stagedRoleChanges": {
+    "e2ee7984ee65b260682086ec70026165903c86e601a4a5a501c1900afe28d84b": {
+      "zone": "dc2",
+      "capacity": 5,
+      "tags": [
+        "node4"
+      ]
+    }
+  }
+}
+```