feat: Allow developer to use raw fields

[MikyChow]

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

What did this pull request do?

User Case Description

f := field.NewRaw("", "DATE_ADD(expired_at, INTERVAL 10 DAY)")

[MikyChow]

This feature wil allow developers to add dynamic columns easily.
for example:

1. case t1.c1 when 1 then a end can be used as a field
2. select * from t1 where t1.c2< t1.c1+200 query by self-referencing

[a67793581]

good

[ilxqx]

Nice, I also have the same requirement, select string_agg(column, ',') ...

I really don't want to write raw SQL.

[foundVanting]

good

[serious-snow]

这个咋还没有合并进去呀

[whit15]

这个pr什么时候能合并啊？开发中确实有时候会需要这个功能

[mimicode]

bad!

[ypli0629]

+1

[ypli0629]

@qqxhb