support different struct in WriteObject

for now, ingester-go only support one type in Client.WriteObject, but it should behave like ingester-java, like:

resp, err := client.WriteObject(context.Background(), cup1, mem1, cup2, mem2)

Function signature

from

func (c *Client) WriteObject(ctx context.Context, obj any) (*gpb.GreptimeResponse, error) 

to

func (c *Client) WriteObject(ctx context.Context, objs ...any) (*gpb.GreptimeResponse, error) 

NOTE

• To simplify the maintenance, WriteObject use variadic parameters