CAY-2597 `case-when` implementation

[stariy95]

PR brings support for the CASE-WHEN SQL operator to the Cayenne expression API.

Usage example (from the test):

Expression caseWhen = ExpressionFactory.caseWhen(
                List.of(Artist.PAINTING_ARRAY.dot(Painting.ESTIMATED_PRICE).between(BigDecimal.ZERO, BigDecimal.valueOf(50)),
                        Artist.PAINTING_ARRAY.dot(Painting.ESTIMATED_PRICE).between(BigDecimal.valueOf(51), BigDecimal.valueOf(100))),
                List.of(Artist.ARTIST_NAME.getExpression(),
                        Artist.DATE_OF_BIRTH.getExpression()));

This PR supersedes #593