docs: add w3 plan basic spec

w3-plan.md

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+# Space
+
+![status:reliable](https://img.shields.io/badge/status-reliable-green.svg?style=flat-square)
+
+## Editors
+
+- [Vasco Santos], [Protocol Labs]
+
+## Authors
+
+- [Vasco Santos], [Protocol Labs]
+
+
+## Abstract
+
+A Plan
+
+- [Capabilities](#capabilities)
+  - [`plan/get`](#planget)
+  - [`plan/set`](#planset)
+
+## Language
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119](https://datatracker.ietf.org/doc/html/rfc2119).
+
+
+## Capabilities
+
+### `plan/get`
+
+Capability can be invoked to get information about the plan that the account is currently signed up for.
+
+> `did:mailto:web.mail:alice` invokes `plan/get` capability provided by `did:web:web3.storage`
+
+```json
+{
+  "iss": "did:mailto:web.mail:alice",
+  "aud": "did:web:web3.storage",
+  "att": [
+    {
+      "with": "did:mailto:web.mail:alice",
+      "can": "plan/get"
+    }
+  ],
+  "prf": [],
+  "sig": "..."
+}
+```
+
+#### Plan Get Failure
+
+```json
+{
+  "ran": "bafy...planGet",
+  "out": {
+    "error": {
+      "name": "PlanNotFound"
+    }
+  }
+}
+```
+
+#### Plan Get Success
+
+```json
+{
+  "ran": "bafy...planGet",
+  "out": {
+    "ok": {
+      "product": "did:web:starter.web3.storage",
+      "updatedAt": "2024-01-05T06:56:26.074Z"
+    }
+  }
+}
+```
+
+### `plan/set`
+
+Capability can be invoked to change a billing plan of an account.
+
+> `did:mailto:web.mail:alice` invokes `plan/set` capability provided by `did:web:web3.storage`
+
+```json
+{
+  "iss": "did:mailto:web.mail:alice",
+  "aud": "did:web:web3.storage",
+  "att": [
+    {
+      "with": "did:mailto:web.mail:alice",
+      "can": "plan/set",
+      "nb": {
+        "product": "did:web:starter.web3.storage"
+      }
+    }
+  ],
+  "prf": [],
+  "sig": "..."
+}
+```
+
+#### Plan Set Failure
+
+```json
+{
+  "ran": "bafy...planSet",
+  "out": {
+    "error": {
+      "name": "AccountNotFound"
+    }
+  }
+}
+```
+
+#### Plan Set Success
+
+```json
+{
+  "ran": "bafy...planSet",
+  "out": {
+    "ok": {}
+  }
+}
+```