[Refactor] Refactor Variable related code to support User Define Variable

[HangyuanLiu]

What type of PR is this：

• [ ] bug
• [ ] feature
• [ ] enhancement
• [x] refactor
• [ ] others

Which issues of this PR fixes ：

Fixes #

Problem Summary(Required) ：

Checklist:

• [x] I have added test cases for my bug fix or my new feature
• [x] I have added user document for my new feature or new function

[HangyuanLiu]

run starrocks_fe_unittest

[sonarqubecloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
9 Code Smells

0.0% Coverage
0.0% Duplication

[HangyuanLiu]

run starrocks_fe_unittest

[wanpengfei-git]

[FE PR Coverage Check]

:disappointed: fail : 56 / 72 (77.78%)

file detail

	path	covered_line	new_line	coverage	not_covered_line_detail
:large_blue_circle:	com/starrocks/analysis/VariableExpr.java	8	18	44.44%	[59, 68, 84, 85, 88, 152, 156, 189, 192, 193]
:large_blue_circle:	com/starrocks/analysis/SetVar.java	24	29	82.76%	[86, 87, 121, 133, 139]
:large_blue_circle:	com/starrocks/sql/ast/SetUserVar.java	8	9	88.89%	[20]
:large_blue_circle:	com/starrocks/analysis/SetType.java	2	2	100.00%	[]
:large_blue_circle:	com/starrocks/sql/parser/AstBuilder.java	10	10	100.00%	[]
:large_blue_circle:	com/starrocks/sql/analyzer/AST2SQL.java	2	2	100.00%	[]
:large_blue_circle:	com/starrocks/qe/VariableMgr.java	2	2	100.00%	[]

[wanpengfei-git]

run starrocks_admit_test