[Enhancement] The decimal creation type is consistent with the display type (backport #38639)

[Astralidea]

Why I'm doing:
When using the following statement to create a table

CREATE TABLE `testdecimal` (
  `obj_id` string NOT NULL COMMENT "",
  `lab_numr` decimal(38, 10) NULL COMMENT ""
) ENGINE=OLAP 
DUPLICATE KEY(`obj_id`)
COMMENT ""
DISTRIBUTED BY HASH(`obj_id`) BUCKETS 10 
PROPERTIES (
"replication_num" = "1",
"in_memory" = "false",
"storage_format" = "DEFAULT",
"enable_persistent_index" = "true"
);

Then after executing show create table

CREATE TABLE `testdecimal` (
  `obj_id` varchar(65533) NOT NULL COMMENT "客户ID",
  `lab_numr` decimal128(38, 10) NULL COMMENT "标签编号"
) ENGINE=OLAP 
DUPLICATE KEY(`obj_id`)
DISTRIBUTED BY HASH(`obj_id`) BUCKETS 10 
PROPERTIES (
"replication_num" = "1",
"in_memory" = "false",
"enable_persistent_index" = "true",
"replicated_storage" = "true",
"fast_schema_evolution" = "true",
"compression" = "LZ4"
);

decimal->decimal128
string->varchar(65533)

The decimal type is relatively stable now, so show create table should remain consistent, while string is just a syntactic sugar, so it should not be changed for the time being.

What I'm doing:
Keep the decimal the same as when created, and the string will not be modified.

Fixes #issue

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 3.2
  • 3.1
  • 3.0
  • 2.5

[github-actions]

[FE Incremental Coverage Report]

✅ pass : 7 / 7 (100.00%)

file detail

	path	covered_line	new_line	coverage	not_covered_line_detail
🔵	com/starrocks/catalog/ArrayType.java	3	3	100.00%	[]
🔵	com/starrocks/catalog/ScalarType.java	1	1	100.00%	[]
🔵	com/starrocks/sql/optimizer/operator/scalar/CastOperator.java	3	3	100.00%	[]

[github-actions]

[BE Incremental Coverage Report]

✅ pass : 0 / 0 (0%)