Add UTS (Universal Test Spec) suite for REST API

[paddybyers]

Summary

• Adds ably-common as a git submodule (pinned to latest main)
• Adds portable, language-independent test specifications covering the REST client API:
  • Auth: auth callbacks, auth scheme, authorize, client ID, token details, token renewal, token request params, revoke tokens
  • Channels: publish, history, annotations, idempotency, message versions, get message, publish result, channel attributes, update/delete messages, channels collection
  • Encoding: message encoding/decoding (JSON, base64, cipher)
  • Batch: batch publish, batch presence
  • Fallback: host selection, retry logic, cache expiry, endpoint routing, option conflicts
  • Presence: REST presence get/history
  • Push: device registrations, channel subscriptions, admin publish
  • Types: error, message, mutable message, options, paginated result, presence message, token types
  • Other: REST client options, request/response, request endpoint, stats, time, logging
  • Helpers: mock HTTP spec
• Includes a README documenting the UTS framework, pseudocode conventions, and mock infrastructure

These specs serve as the source of truth for expected SDK behaviour, independent of any specific programming language implementation. Each spec references the relevant specification points (e.g. RSA, RSL, RSC) and describes test setup, actions, and assertions in portable pseudocode.

Test plan

• Review specs for consistency with features.md
• Verify pseudocode is language-agnostic and follows conventions in README

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