feat(AIP-123): require singular and plural (#1091)

Requiring singular and plural annotations on resources
can enable new use cases in code generation and documentation,
and prevents incorrect inferences about these values from
the resource type.

See googleapis/api-linter#722 for
additional discussion around the usage of singular and plural.

Author: toumorokoshi

aip/general/0123.md

@@ -63,6 +63,8 @@ message Topic {
   option (google.api.resource) = {
     type: "pubsub.googleapis.com/Topic"
     pattern: "projects/{project}/topics/{topic}"
+    singular: "topic"
+    plural: "topics"
   };
 
   // The resource name of the topic.
@@ -75,9 +77,13 @@ message Topic {
 - Patterns **must** correspond to the [resource name][aip-122].
 - Pattern variables (the segments within braces) **must** use `snake_case`, and
   **must not** use an `_id` suffix.
-- Pattern variables **must** be the singular form of the resource type e.g. the
-  pattern variable representing a `Topic` resource ID is named `{topic}`.
 - Pattern variables **must** conform to the format `[a-z][_a-z0-9]*[a-z0-9]`.
+- Singular **must** be the lower camel case of the type.
+  - Pattern variables **must** be the singular form of the resource type e.g.
+    a pattern variable representing a `Topic` resource ID is named `{topic}`.
+- Plural **must** be the lower camel case plural of the singular.
+  - Pattern collection identifier segments **must** match the plural of the
+    resources, except in the case of [nested collections][].
 
 #### Pattern uniqueness
 
@@ -92,16 +98,28 @@ resource:
 - `user/{user}`
 - `user/{user_part_1}~{user_part_2}`
 
+## Rationale
+
+### Singular and Plural
+
+Well-defined singular and plurals of a resource enable clients to determine the
+proper name to use in code and documentation.
+
+lowerCamelCase can be translated into other common forms of a resource name
+such as UpperCamelCase and snake_case.
+
 <!-- prettier-ignore-start -->
 [aip-122]: ./0122.md
 [API Group]: https://kubernetes.io/docs/concepts/overview/kubernetes-api/#api-groups
+[nested collections]: ./0122.md#collection-identifiers
 [Object]: https://github.com/kubernetes/community/blob/master/contributors/devel/sig-architecture/api-conventions.md#types-kinds
 [resource]: https://github.com/googleapis/googleapis/blob/master/google/api/resource.proto
 [service configuration]: https://github.com/googleapis/googleapis/blob/master/google/api/service.proto
 <!-- prettier-ignore-end -->
 
 ## Changelog
 
+- **2023-05-06**: Adding requirement of singular and plural.
 - **2023-01-28**: Clarifying guidance for the resource type name.
 - **2022-10-28**: Added pattern variable format guidance.
 - **2020-05-14**: Added pattern uniqueness.