feat: add Only

[mo1ein]

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

What did this pull request do?

Introduces a new Only query method (see issue #7411). This method retrieves exactly one record ordered by primary key and returns an error if zero or multiple records match.

User Case Description

GORM previously lacked a built‑in Only operation, forcing users to manually combine First and Count to enforce uniqueness. Many other ORMs provide this feature natively.

[jinzhu]

thank you for your PR! However, we’d prefer not to add too many “sugar” APIs.

And a better implementation might be using LIMIT 2 to check if there’s more than one record, rather than triggering an additional COUNT query.

thank you

[mo1ein]

So shouldn't the issue be closed too?