Improvements to work with table relations, added support to associations such as oneToMany and manyToOne

[brunomaraujo]

I've added support to associations such as manyToOne and oneToMany, changing the classes FilterExpressionParser and JpaSpecificationExpressionVisitor to use in my own projects.

The modifications also include :

• Added support to is null or is not null filters in JpaSpecificationExpressionVisitor
• Commented the method convertIfDate to allow the use of date types other than Date, such as OffsetDateTime, LocalDateTime or LocalDate.

I'd like to contribute to this project with these improvements.

This is an example project I've made using these improvements.
graphql-dynamicquery.zip

[JOOmsH]

Hello,

Thank you for your improvements; I found them very useful. Is there a good way to map the model variableName (linked table) with the name of the input object in GraphQL? I would prefer not to name my linked table the same as the input object :)