[C++] Extract reusable simdjson helper utilities #50746

Describe the enhancement requested

Some generic simdjson helper functions are currently local to from_string.cc but are useful in other simdjson-based implementations as well.

Extract the reusable helpers into a shared utility (e.g. arrow/util/simdjson_internal.h).

Related: #50725 (comment)

Address #50725 (comment) as well.

Component(s)

C++