Add multi model config

[AlbertChenshiqi]

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

What did this pull request do?

通常情况下，多张业务逻辑表有多个分表的维度。例如，订单表可以按订单ID分表；同样，按用户ID分表，在根据用户ID查询订单时也能提速，就像我们常查看自己的订单那样。这个组件原来也是支持这种功能，只不过需要初始化不同的数据库实例才能实现。为了更好的支持这个功能：赋能单表，灵活支持多分表策略，我基于model实现多model的register。

User Case Description

support multiple config with multiple model register
model must implement ShardingInterface
example: base_test.go

type Order struct {
    gorm.Model
    UserID    uint64
    ProductID int64
    Amount    int64
    sharding.BaseSharding
}

func (Order) TableName() string {
    return "order"
}

type OtherTable struct {
    gorm.Model
    UserID    uint64
    
    sharding.BaseSharding
}

func (OtherTable) TableName() string {
    return "other_table"
}
otherTable := &OtherTable{}
otherTable.DefaultShards = 2
db.Use(sharding.RegisterWithModel(&Order{}, otherTable))