Add public BuildCondition method for associations

[greencoda]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

This PR proposes to add a public variant of the buildCondition() method, as it would allow dynamic query creation, for cases when the user would like do additional filtering, ordering or limiting before calling Find() or Count() on the transaction statement.

User Case Description

I'm creating a connector between GORM and GraphQL, where associated fields on a model are returned as the result of a pagination function, which allows the user to apply ORDER, LIMIT and WHERE conditions before returning a page of associated records.

As the association struct does not allow for setting these before calling the Find() and Count() methods, having access to this private buildCondition() method is the next best way to be able to construct this query.