[Enhancement] Scale Backends for connector sink when writing static partition tables

[mxdzs0612]

Why I'm doing:

What I'm doing:

By default, all backends will take part in connector sinking all from the start, leading to small files when there is little data. This PR scales up backends one by one for static partition tables, which can cope with this problem.

todo: add result pictures after #40540 is merged.

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

[sonarqubecloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[github-actions]

[FE Incremental Coverage Report]

✅ pass : 20 / 23 (86.96%)

file detail

	path	covered_line	new_line	coverage	not_covered_line_detail
🔵	com/starrocks/qe/SessionVariable.java	2	4	50.00%	[2622, 3482]
🔵	com/starrocks/sql/InsertPlanner.java	11	12	91.67%	[804]
🔵	com/starrocks/catalog/HiveTable.java	4	4	100.00%	[]
🔵	com/starrocks/planner/DataPartition.java	2	2	100.00%	[]
🔵	com/starrocks/sql/plan/PlanFragmentBuilder.java	1	1	100.00%	[]

[github-actions]

[BE Incremental Coverage Report]

✅ pass : 38 / 42 (90.48%)

file detail

	path	covered_line	new_line	coverage	not_covered_line_detail
🔵	be/src/exec/pipeline/exchange/exchange_sink_operator.cpp	36	40	90.00%	[569, 572, 574, 589]
🔵	be/src/exec/pipeline/exchange/sink_buffer.h	2	2	100.00%	[]