feat: allow priority tag in composite PK

[yaofeng-wang]

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

What did this pull request do?

Fixes issue#6755 to allow priority tag for composite primary key, similar to unique index.

User Case Description

type Bad struct {
	A   ItemA
	AId uint `gorm:"primaryKey,priority:2;autoIncrement:false"`
	B   ItemB
	BId uint `gorm:"primaryKey,priority:1;autoIncrement:false"`
}

Composite primary key will be created and it follows ordering specified by priority tag:

[jinzhu]

Why not use a unique index? Doesn't it seem more intuitive?

[yaofeng-wang]

Why not use a unique index? Doesn't it seem more intuitive?

I think it's better to be able to choose between composite primary key and unique index.

[jinzhu]

We're planning to close this PR because we aim to minimize the complexity introduced by some edge cases. If necessary, we can ensure that this field is ordered in the model definition.

Thank you for your PR.