Infer TS type from JSON schema

[cyrilletuzi]

Description of the issue

Currently the lib infers the TS type in get/watch from the JSON schema for simple types (string, number, boolean and array of the former), but not for complex arrays or objects, leading to:

• a double work for the user, who must do the type and the JSON schema
• we are not sure the type match the JSON schema

It is now possible with today's TypeScript to infer the type from the JSON schema even for complex types.

Note it requires to assert JSON schemas as const (maybe microsoft/TypeScript#30680 could help, but it doesn't seem it will get a solution from TS).

[cyrilletuzi]

PR #477

Blocked on Angular supporting TS 4.1 (and TS >= 4.1 only) for recursive conditional types, so probably not before Angular v12.

Also, a DX issue (no IntelliSense when writing the JSON schemas) is discussed in microsoft/TypeScript#41247, but it doesn't seem it will get a solution from TS.