Make coercion and validation work off of @loopback/types

[shimks]

Description / Steps to reproduce / Feature proposal

For DP3, we'd like validation and coercion system implemented in the http-layer to use @loopback/types

Acceptance Criteria

• Move types folder from @loopback/repository into its own package; Make sure this PR is refined and make sure it lands [WIP] feat: add @loopback/types to be the typing system #947
• Move over the converters used in http coercion into appropriate files in @loopback/types with extensibility in mind.
  • Come up with a decorator system (@param or a new one if necessary) to introduce metadata to appropriately infer type metadata (for example Date)

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[bajtos]

@shimks we already have another story covering validation at REST layer, see #750:

Acceptance Criteria

• Coerce incoming query string, path and header inputs into strings, numbers, booleans, Dates
  • Do not coerce timestamps (Unix timestamps in milliseconds)
• Validate only that the output is correct for the chosen type

Notes

We should consider using either strong-remoting or AJV to perform the coercion

Assuming we implement #750 first, what would be the scope & acceptance criteria of this story #1319?

[shimks]

@bajtos I believe the acceptance criteria should remain unchanged considering the scope of this task is to move over the converters used in http coercion into a 'LoopBack' typing hub. It might have been more appropriate to have changed the title to exclude 'validation', but there may be some overlap between the two and if there is, it should be small enough of a change to implement in one PR.

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