Merge DialectSchemaMetaDataLoader#loadSchemaNames to SchemaMetaDataLoader#loadSchemaNames method

[strongduanmu]

Changes proposed in this pull request:

• merge DialectSchemaMetaDataLoader#loadSchemaNames to SchemaMetaDataLoader#loadSchemaNames method
• adjust private method sequence in OpenGaussSchemaMetaDataLoader and PostgreSQLSchemaMetaDataLoader
• add unit test for SchemaMetaDataLoader#loadSchemaNames

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• 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 #22351 (c6bd8aa) into master (77c234b) will increase coverage by 0.01%.
The diff coverage is 62.09%.

@@             Coverage Diff              @@
##             master   #22351      +/-   ##
============================================
+ Coverage     51.02%   51.04%   +0.01%     
+ Complexity     2508     2506       -2     
============================================
  Files          4164     4163       -1     
  Lines         57838    57817      -21     
  Branches       9822     9814       -8     
============================================
  Hits          29510    29510              
+ Misses        25786    25774      -12     
+ Partials       2542     2533       -9     

Impacted Files	Coverage Δ
.../loader/dialect/OpenGaussSchemaMetaDataLoader.java	2.94% <0.00%> (+0.16%)	⬆️
...ase/schema/loader/common/SchemaMetaDataLoader.java	86.66% <92.85%> (ø)
...loader/dialect/PostgreSQLSchemaMetaDataLoader.java	90.52% <95.45%> (+3.14%)	⬆️
...ingletable/datanode/SingleTableDataNodeLoader.java	92.85% <100.00%> (ø)
...handler/distsql/ral/hint/enums/HintSourceType.java	42.85% <0.00%> (+42.85%)	⬆️

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