Add derived column along with default fields

[ThomasP1988]

Describe the feature

Being able to add a derived column along with the default ones generated by Model or Dest (and joined tables)

Motivation

In my case, I would like to get a similary score with pgvector, however I would like to get it along with my model and its relationships. Select is erasing the default model querying if you add anything else than * in the function.

here is the original query I wanted to achieve:

articles := []models.AssetArticle{}
	 db.Select("*, 1 - (embedding <-> ?) AS similarity_score").Model(models.AssetArticle{}).Joins("Publisher").Omit("Embedding", "Log").Find(&articles)

the problem with this one is that Publisher won't be populated by the result.

Failed attempts:

• I tried to first use a dry run query and copy the select clause to edit it but it is erased after the query is processed
• I tried to write a callback but after digging into the query callback, i discovered that the whole processing is done and the query executed right after, preventing any mutation in the between

I'm happy to make a PR if you imagine a correct implementation of this issue.