Add support for DATE type (values with a date part but no time part)

[bajtos]

Based on the discussion in #1948, I am proposing to introduce a new type to represent SQL DATE values (a value with a date part but no time part).

• We should probably introduce a new interface or class to represent these values.
• Internally, dates should be stored as strings to avoid a whole set of timezone-related problems.
• JSON Schema/OpenAPI schema should use {type: 'string', format: 'date'}
• Connectors should recognize this new type at runtime, during discovery and autoupdate too.
• We may need to configure the connectors to return DATE values as strings (not JavaScript Date instances). See MySQL connector's option dateStrings for an example.

Related work: DateString type provided by juggler. See loopbackio/loopback-datasource-juggler#1356, loopbackio/loopback-datasource-juggler#1365, the api docs and a possible bug loopbackio/loopback-datasource-juggler#1636. Please note that DateString is not the solution we are looking for, at least not in the current form! (See the discussion in #1948 for explanation.)

[bajtos]

Heads up: it may be difficult to support DATE in PostgreSQL, it seems like DATE values are always converted to JavaScript's Date. See brianc/node-postgres#1844, brianc/node-postgres#783, brianc/node-postgres#510 and linked issues/pull-requests.

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