Add ability to generate several sort expressions for one property using JpqlSortExpressionProvider

[dtaimanov]

see forum topic

Now it is expected that io.jmix.data.persistence.JpqlSortExpressionProvider#getDatatypeSortExpression will return single sort expression and all expressions after , are ignored by QueryTransformerAstBased.

Task:

1. Add ability to return several sort expression strings from io.jmix.data.impl.jpql.generator.SortJpqlGenerator#getDatatypePropertySortExpression splitting them by ,
2. Clarify in javadocs for io.jmix.data.persistence.JpqlSortExpressionProvider#getDatatypeSortExpression` that sort direction should not be included in expression