feat: support comparison operators for missing edm types

[ZhongpinWang]

Closes SAP/cloud-sdk-backlog#488.

Edm.String, Edm.Boolean, and Edm.Guid are primitive OData types and should support all six operations. Thus, they are changed from EdmTypeField to OrderableEdmTypeField.

I also added lt, le, gt, ge for the string filter function, which makes, e.g., the following case possible

filterFunctions().substring('abcde', 1).greaterOrEqual('bcde');

[FrankEssenberger]

Perhaps I have overseen it but could you add tests that for the primitive types the filter which are supposed to be present are also there.