[Enhancement] Removes reserve() from array_agg(). (backport #56958)

[mergify]

Why I'm doing:

While debugging a slow query which took a few minutes, we found that it was due to skewed data.
But it was not clear why the skewed data caused such a long latency. After further investigation, I found that it was due to unnecessary reserve() calls in array_agg().

After appending a large array to a result column in one aggregated row, following finalize_to_column()s even with a small number of elements took a few ms likely to allocate new memory and copy existing values. Thousands of aggregated rows with a few ms added a lot of latency.
Repeatedly calling reserve() with small increases is harmful. Without reserve(), in the following append calls, std::vector would be able to increase the capacity exponentially efficiently.

What I'm doing:

Removes reserve().

In a test on skewed data, a latency of a query with array_agg() decreased from 2m45s to 21s.

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