LiveObjects : Setup liveobject tests

[sacOO7]

Spec : ably/specification#298

Resources

Roadmap: https://ably.atlassian.net/wiki/spaces/LOB/pages/3015672242/Live+Objects+Product+Roadmap
Touchtunes proposal (indicative of client API): https://docs.google.com/document/d/1Sph98VPvPw5E6mBWMDtTVOJyL5_cKCVnPrUx2BznqxI/edit
CRDT experiments: https://github.com/ably-labs/crdts/tree/object-pool/ts/object-pool
DRs: https://ably.atlassian.net/wiki/spaces/LOB/pages/3090219015/Decision+Records+Live+Objects+LODRs

Spec PR

ably/specification#298

┆Issue is synchronized with this Jira Task by Unito

[sacOO7]

1. Add annotation for liveobject plugin-specific code
2. Add serializer interface