fix: preserve Utf8View return type in lpad

[Tim-53]

Which issue does this PR close?

Part of #20585

Rationale for this change

lpad was returning Utf8 when given Utf8View inputs, causing unnecessary type casting downstream

What changes are included in this PR?

• Fix return_type to preserve Utf8View when the input is Utf8View
• Introduce StringArrayWriter trait in unicode/common.rs to abstract over GenericStringBuilder
  and StringViewBuilder, allowing lpad to produce the correct output type
• Add tests covering Utf8View return type and value correctness in string_view.slt

Are these changes tested?

Yes new unit tests were added to cover all input/fill combinations.
Slt tests were added to verify the return type is preserved for Utf8View inputs

Are there any user-facing changes?

lpad now correctly returns Utf8View when given Utf8View input.

Note: rpad has the same issue and will be addressed in a follow-up.