META: Selection subscription API

[lmeyerov]

Expose single/multi-selection events to parent frame

• Internal implementation: Index translation, ... https://github.com/graphistry/graphistry/issues/1861
• Performance optimization: Pay-as-you-go (global) https://github.com/graphistry/graphistry/issues/1862
• Performance optimization: Client/server caching https://github.com/graphistry/graphistry/issues/1863
• Enrichment with ID, labels
• Enable selection API for feature-flagged users graphistry-js#109
• Expose new selection API graphistry-js#108
• https://github.com/graphistry/graphistry/issues/1860
• https://github.com/graphistry/graphistry/issues/1866
• Selection read API demo & docs graphistry-js#113
• (TBD: Bugs!)