Support user level sql function

[pythonbrad]

Linked to

#660

TODO

• Show function statement
• Drop function statement
• Create function statement

Example

CREATE FUNCTION nothing();
CREATE FUNCTION hello() RETURN 'hello!';
CREATE FUNCTION add(x int) RETURN x+1;
CREATE FUNCTION div(a int, b int) RETURN a/b;
CREATE FUNCTION incr(a int, b int default 1) RETURN (SELECT bar from foo);
SELECT nothing();
SELECT incr(1);
SHOW FUNCTIONS;

[coveralls]

Pull Request Test Coverage Report for Build 4677170930

• 526 of 539 (97.59%) changed or added relevant lines in 20 files are covered.
• 28 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.02%) to 98.874%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/src/executor/alter/function.rs	33	34	97.06%
core/src/executor/execute.rs	18	20	90.0%
core/src/store/function.rs	10	20	50.0%

Files with Coverage Reduction	New Missed Lines	%
core/src/ast/function.rs	1	99.87%
core/src/ast/mod.rs	1	99.83%
core/src/plan/expr/function.rs	1	99.55%
core/src/executor/aggregate/mod.rs	2	86.53%
core/src/executor/evaluate/function.rs	2	98.93%
core/src/executor/evaluate/stateless.rs	2	94.01%
core/src/executor/execute.rs	19	91.86%

Totals
Change from base Build 4636876365:	0.02%
Covered Lines:	43042
Relevant Lines:	43532

---

💛 - Coveralls

[panarch]

Wow, now it's really close to the completion!

With some comments, it would be nice that we can add a few more tests to reach uncovered lines.
ref. you can ignore await uncovered lines which we cannot make it work.
https://coveralls.io/builds/57904316

[ever0de]

Suggested change

	pub mod function;
	mod function;

[ever0de]

Suggested change

	expected_minimum: usize,
	expected_maximum: usize,
	expected_range: Range<usize>

[panarch]

@ever0de current custom function does not support open range so it would be better to use expected_minimum & expected_maximum.
Range can describe (3..) or (..=4) like open ranges that are not supported by our custom function args

[pythonbrad]

@ever0de , finally i done a rebase. I apply reviews.

[ever0de]

Could you please confirm and apply CI?

[pythonbrad]

@panarch, i fixed it

[panarch]

Ok now it's really the time to ship this awesome new feature to the main!
Looks all great 👍 👍 👍
Thanks a lot!