Add create index concurrently option

[s-takehana]

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

What did this pull request do?

Related to go-gorm/gorm#4423.

https://www.postgresql.org/docs/current/sql-createindex.html

User Case Description

type User struct {
	gorm.Model
	Name string `gorm:"index:,option:CONCURRENTLY"`
}

2021/05/19 03:56:58 /go/pkg/mod/gorm.io/driver/postgres@v1.0.8/migrator.go:134
[6.462ms] [rows:0] CREATE INDEX CONCURRENTLY "idx_users_name" ON "users" ("name")

[jinzhu]

thank you for your PR.