Add guidance that keys with the '/' character should be base64url encoded

graph/patterns/encoded-keys.md

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+# Encoded Key Properties
+
+Microsoft Graph API Design Pattern
+
+Key properties should be encoded with [base64url encoding](https://datatracker.ietf.org/doc/html/rfc4648#section-5) when their values can include the `/` character. 
+
+
+## Problem
+
+OData URLs that identify an individual entity within a collection will contain the key for that entity.
+These keys can sometimes include reserved URL characters, and the [OData standard](https://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part2-url-conventions.html#sec_URLParsing) indicates that these characters should be percent-encoded. 
+Due to a long-standing bug in Microsoft Graph, and one that cannot be fixed in order to maintain backwards compatibility, percent-encoding the `/` character is not possible for Microsoft Graph APIs.
+This means that key properties cannot contain the `/` character, which can be limiting to the API design.
+
+## Solution
+
+A key property that may contain the `/` character should be base64url encoded, and each instance of the property should be encoded regardless if that particular value contains the `/` character.
+The entity may additionally include a non-key property that contains the plaintext of the key property.
+This property should have the same name as the key property with `Plaintext` appended. 
+The collection may also support filtering on the plaintext property.
+
+## When to use this pattern
+
+This pattern should be used whenever the value of a key property may contain a `/` character.
+
+## Issues and considerations
+
+Encoding a key property often obfuscates the natural way that a client identifies an entity, particularly entities that are related to extenal services or standards.
+A plaintext property that allows filtering can help to reduce this impact, but filtering itself has semantic limitations in OData (for example, if the client wants to navigate to a navigation property of the entity, using a filter does not allow this).
+
+## Example
+
+```xml
+<EntityType Name="foo">
+  <Key>
+    <PropertyRef Name="id" />
+  </Key>
+  <Property Name="id" Type="Edm.String" Nullable="false">
+    <Annotation Term="Org.OData.Core.V1.Description" String="The base64url encoding of the idPlaintext property" />
+  </Property>
+  <Property Name="idPlaintext" Type="Edm.String" Nullable="false" />
+</EntityType>
+```
+```http
+GET /foos/dGhpcyBpcyBhbiBpZCB3aXRoIC8
+
+200 OK
+{
+  "id": "dGhpcyBpcyBhbiBpZCB3aXRoIC8",
+  "idPlaintext": "this is an id with /"
+}
+```
+```http
+GET /foos('dGhpcyBpcyBhbiBpZCB3aXRoIC8')
+
+200 OK
+{
+  "id": "dGhpcyBpcyBhbiBpZCB3aXRoIC8",
+  "idPlaintext": "this is an id with /"
+}
+```
+```http
+GET /foos?$filter=idPlaintext eq 'this is an id with /'
+
+200 OK
+{
+  "value": [
+    {
+      "id": "dGhpcyBpcyBhbiBpZCB3aXRoIC8",
+      "idPlaintext": "this is an id with /"
+    }
+  ]
+}
+```