[EPIC] Validation and Coercion (4.0 GA)

[jannyHou]

This epic continues where the previous one finished: #755.

Support "new user sign up" story. Interesting cases: the provided email address must be unique and also a valid email per RFC spec.

• Validate and coerce most OpenAPI parameter/data types
• Validate (and coerce?) complex request bodies as described by JSON Schema subset supported by OpenAPI.

Scope

P1

• Router: coercion of complex parameters from string sources #100 Router: coercion of complex parameters from string sources
• Generate OpenAPI schema for Model properties of built-in JavaScript/Node.js types #1112 Generate OpenAPI schema for Model properties of built-in JavaScript/Node.js types
• Cache pre-compiled AJV validators #1590 Cache pre-compiled AJV validators

P2

• Make coercion and validation work off of @loopback/types #1319 make coercion and validation work off of @loopback/types
• [SPIKE] Resolve $ref in api/controller/route specs #435 Spike: Resolve $ref in api/controller/route specs