Channel Management API: Create, Delete, Sync, and Query

A new query, channelByHandle, lets apps retrieve a Channel by its unique handle. Four mutations—channelCreate, channelUpdate, channelDelete, and channelFullSync—provide full CRUD and full‑sync capabilities, each returning payloads with detailed userErrors and trace information. Pagination support is added for App.channels and AppCatalog.channels via a ChannelConnection type, enabling efficient listing of channel connections. Channel objects now expose accountId, accountName, specificationHandle, and a resourceFeedback link. Feedback types (AppFeedback and ProductResourceFeedback) now include a channel field. ProductFeed gets a required channelId field to associate feeds with channels. All new input objects (ChannelCreateInput, ChannelUpdateInput) require specificationHandle, accountId, and accountName, and support optional handle overrides and restoration flags. Developers should update their schemas to use the new fields and mutations for full channel lifecycle management.