Tracking issue for new Schema API

[Frando]

A tracking issue for the new Schema API.

High-level overview:

• Add a schema module in sonar-common that is available to both the client and the schema
• In the server, the core queries the island for all schemas, adds additional schemas defined in source code, and adds them to the schema module
• In the client, when opening an island, the client asks the server for all schemas and adds to its instance of the schema module
• The schema module exposes functions to decompose and map a record
  • decompose(record) returns an array of all (field, value) tuples of the record, with the field schema readily available (containing type, indexing properties, metadata)
  • validate(record) validate a record against its schema
  • map(record, targetSchema) try to map a record onto a targetSchema
• The schema module might also need some ways to work with out CRDT - or at least a way to decompose/reduce a list of records with the same ID onto a target schema
• We'll also want to revisit our schema definition. Internally, we'll want to define a format we own for the schema definition.
  • import/export from/to JSON schema
  • import/export into a more-simple toml

[Frando]

This is mostly done. Documentation is mostly still missing though.