feat: support embedded preload

[black-06]

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

What did this pull request do?

Support embedded preload, especially the same structure. see #6131

Unimplemented:

• Polymorphism has_one/has_many in repeating embedded struct // Their polymorphic value in db are all the same.

User Case Description

type Country struct {
	ID   int `gorm:"primaryKey"`
	Name string
}
type Address struct {
	CountryID int
	Country   Country
}
type NestedAddress struct {
	Address
}
type Org struct {
	ID              int
	PostalAddress   Address `gorm:"embedded;embeddedPrefix:postal_address_"`
	VisitingAddress Address `gorm:"embedded;embeddedPrefix:visiting_address_"`
	AddressID       int
	Address         struct {
		ID int
		Address
	}
	NestedAddress *NestedAddress `gorm:"embedded;embeddedPrefix:nested_address_"`
}

// Only preload Org.Address and Org.Address.Country
db.Preload("Address.Country")  // "Address" is has_one, "Country" is belongs_to (nested relation)

// Only preload Org.VisitingAddress
db.Preload("PostalAddress.Country") // "PostalAddress.Country" is belongs_to (embedded relation)

// Only preload Org.NestedAddress
db.Preload("NestedAddress.Address.Country") // "NestedAddress.Address.Country" is belongs_to (embedded relation)

// All preloaded include "Address" but exclude "Address.Country", because it won’t preload nested associations.
db.Preload(clause.Associations)