feature: Settings clause

[sundy-li]

Summary

syntax

SETTINGS ( parameter = value [ , parameter = value ] )  Statement

SETTINGS clause works only in Statement of:

SELECT
INSERT 
MergeINTO
COPY
UPDATE
DELETE
CREATE TABLE 
EXPLAIN

examples:

settings (max_thread = 3,  enable_loser_tree_merge_sort = 1) select  1 + 1 

settings (max_thread = 3,  enable_loser_tree_merge_sort = 1)  copy into ....

We already had set_var_hints now, but the syntax is not friendly. Settings could also be parsed into HintItem.

Any advice? cc @zhang2014 @andylokandy

Refer to https://docs.yellowbrick.com/6.8.1/ybd_sqlref/setting_clause.html

[BohuTANG]

Why not use set_var to do it?

[zhang2014]

LGTM

[TCeason]

Maybe add a new type in SetType is better than HintItem?

pub enum SetType {
    #[default]
    SettingsSession,
    SettingsGlobal,
    Variable,
    SettingsQuery,
}

And if user use query level settings and hints , the hints will be invalid.

That's easy to delete the code about hints.