fix: allow overriding existing meta

[ypli0629]

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

What did this pull request do?

allow overriding existing meta

User Case Description

g.ApplyBasic(g.GenerateAllTable()...)

permissions := g.GenerateModel("sys_permissions")
roles := g.GenerateModel("sys_roles", gen.FieldRelate(field.Many2Many, "Permissions", permissions,
  &field.RelateConfig{
	GORMTag: field.GormTag{
		"many2many": []string{"role_permissions"},
	},
}))
g.ApplyBasic(roles)