perf: Optimize starts_with and ends_with for scalar arguments

[andygrove]

Which issue does this PR close?

• Closes #.

Rationale for this change

• Scalar argument optimization delivers 3.6x-8x speedup for the common case of starts_with(column, 'literal') or ends_with(column, 'literal')
• StringViewArray benefits even more (~6-8x) than StringArray (~3.6-3.8x)
• The optimization uses Arrow's Scalar wrapper to avoid broadcasting scalar values to full arrays

starts_with

Benchmark	Before	After	Speedup
StringArray + scalar	32.38 µs	8.49 µs	3.8x
StringViewArray + scalar	78.15 µs	9.82 µs	8.0x

ends_with

Benchmark	Before	After	Speedup
StringArray + scalar	32.76 µs	9.06 µs	3.6x
StringViewArray + scalar	76.44 µs	12.04 µs	6.4x

What changes are included in this PR?

Handle all combinations of array and scalar arguments without converting scalars to arrays

Are these changes tested?

Yes, new unit tests added in this PR.

Are there any user-facing changes?

No, just faster performance.