Add IF NOT EXISTS to register storage unit statement

[Qianyi951015]

The first task of #22844.

Changes proposed in this pull request:

• Add IF NOT EXISTS to the register storage unit statement

---

Before committing this PR, I'm sure that I have checked the following options:

• My code follows the code of conduct of this project.
• I have self-reviewed the commit code.
• I have (or in comment I request) added corresponding labels for the pull request.
• I have passed maven check locally : ./mvnw clean install -B -T1C -Dmaven.javadoc.skip -Dmaven.jacoco.skip -e.
• I have made corresponding changes to the documentation.
• I have added corresponding unit tests for my changes.

[codecov-commenter]

Codecov Report

Merging #23066 (13c2f3a) into master (7677c79) will decrease coverage by 0.00%.
The diff coverage is 41.66%.

@@             Coverage Diff              @@
##             master   #23066      +/-   ##
============================================
- Coverage     50.03%   50.02%   -0.01%     
  Complexity     2424     2424              
============================================
  Files          4110     4110              
  Lines         57354    57362       +8     
  Branches       9048     9054       +6     
============================================
+ Hits          28695    28697       +2     
- Misses        26149    26154       +5     
- Partials       2510     2511       +1     

Impacted Files	Coverage Δ
...ser/core/kernel/KernelDistSQLStatementVisitor.java	0.00% <0.00%> (ø)
...dl/resource/RegisterStorageUnitBackendHandler.java	79.54% <45.45%> (-11.89%)	⬇️
...rnal/cases/parser/jaxb/RootSQLParserTestCases.java	99.07% <0.00%> (-0.01%)	⬇️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more