feat(compute): add list_element

[fallintoplace]

Rationale for this change

Arrow-Go supports the list, large-list, list-view, and fixed-size-list array types, but callers currently need to read offsets and child arrays themselves to select one element from every row. A list_element kernel provides the common nested-data operation directly through compute.

What changes are included in this PR?

• Add list_element for List, LargeList, ListView, LargeListView, and FixedSizeList.
• Accept signed or unsigned integer indexes, including a one-element integer array used as the index.
• Return an array of the list element type while preserving parent nulls and child nulls.
• Return errors for null or negative indexes, empty lists, and indexes outside the length of a valid list.
• Use the existing take kernels for primitive, binary, and fixed-width child values to avoid one array allocation per input row. Keep the generic concatenate path for nested child values.
• Handle list-view offsets and sizes without requiring the input to be materialized first.

Are these changes tested?

• go test ./arrow/compute/... -count=1
• Added coverage for sliced List and FixedSizeList inputs, non-contiguous ListView offsets, unsigned indexes, string and nested-list children, parent and child nulls, and invalid indexes.
• Added BenchmarkListElement cases for 1K, 100K, and 1M rows with allocation reporting.

Are there any user-facing changes?

Yes. This adds the public compute.ListElement function. Existing functions and APIs are unchanged.

[zeroshade]

The list_element implementation has broad test coverage and the compute, array, and targeted race suites pass. However, I reproduced a blocking public API panic for an unsupported scalar first argument; this should return a dispatch/type error instead.

[zeroshade]

Blocking: This unchecked assertion makes unsupported scalar inputs panic instead of returning the normal dispatch/type error. For example, ListElement(ctx, ScalarDatum(Int32(7)), ScalarDatum(Int64(0))) panics with *scalar.Int32 is not scalar.ListScalar; the equivalent unsupported array input returns an error. Please guard the list-like/type assertion and either fall through to normal kernel dispatch or return arrow.ErrType, with a regression test for a non-list scalar argument.