feat: Add workspace plugin sharing APIs (#20278)

1. Adds v2 plugin/share/save, plugin/share/list, and plugin/share/delete
RPCs.
2. Implements save by archiving a local plugin root, enforcing a size
limit, uploading through the workspace upload flow, and supporting
updates via remotePluginId.
3. Lists created workspace plugins
4. Deletes a previously uploaded/shared plugin.

codex-rs/app-server-protocol/schema/json/v2/PluginShareDeleteResponse.json

@@ -0,0 +1,5 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "PluginShareDeleteResponse",
+  "type": "object"
+}