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

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](https://github.com/intuit/graphql-filter-java/files/10093267/graphql-dynamicquery.zip)