Allow coercion and validation to be extensible

[shimks]

Description / Steps to reproduce / Feature proposal

Acceptance Criteria

• to be hashed out after the completion of Make coercion and validation work off of @loopback/types #1319, Use OpenAPI v3 spec to coerce primitives in query, path and headers #750 and Router: validate input parameters #118

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[bajtos]

A spike for 4.0 GA: #1463

[shimks]

Closing in favor of #1463.