expression: add json_arrayagg and json_objectagg

[Kingwl]

What problem does this PR solve?

#7623
#7546

What is changed and how it works?

add an aggr function

Check List

Tests

• Unit test

---

This change is 

[sre-bot]

Hi contributor, thanks for your PR.

This patch needs to be approved by someone of admins. They should reply with "/ok-to-test" to accept this PR for running test automatically.

[shenli]

@Kingwl Thanks!

[Kingwl]

@shenli could you give some help, please😂

[zz-jason]

@Kingwl What problem do you encountered?

[Kingwl]

where should I add the test?
or only 'integration_test'?

[zz-jason]

@Kingwl how about adding test in file executor/aggregate_test.go?

[Kingwl]

@zz-jason seems a big work, could I add the JSON agg function only?😂

[zz-jason]

@Kingwl sure, it’s also the recommended way. 😄

[Kingwl]

and the pingcap/tipb#105 has blocked this (and #7776) , could you take a look please?🙋🏻‍♂️

[Kingwl]

And something can not to be serialized to binary JSON (Decimal), are there any existed way to handle that?

[zz-jason]

@Kingwl Why decimal can not be serialized to binary json?

[Kingwl]

json.CreateBinary does not match that, maybe convert all value to []byte? (or string? or any other type)

[Kingwl]

another question:
what kind should decimal be in json? number ? or string?

[Kingwl]

@ PTAL @zz-jason

[qw4990]

Hi, @Kingwl, could you please fix these CI problems and conflicts? Thanks~

[Kingwl]

sure

[qw4990]

PTAL~ @Kingwl

[zz-jason]

@Kingwl friendly ping, any update?

[Kingwl]

Sorry for the delay，I have no time to update with these recently. And the changes seem not completion, Sorry again.