util: Add dedicated hex conversion functions and use it.

util: Add dedicated hex conversion functions and use it.

This deduplicate two identical bytes_to_hex implementation into one.

The intention is to ease the introduction of a new hash algorithm, which
will also have its formatting helper (to ensure seamless transition from
sha1).

Note that the new functions always take the size of the binary buffer,
unlike the old disk_cache_format_hex_id which took `binary * 2` which was
inconsistent (binary size is `binary` and string size is `binary * 2 + 1`).

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22527>