feat(spark): add to_binary and try_to_binary

[arsathS]

Which issue does this PR close?

• Part of [EPIC] Complete datafusion-spark Spark Compatible Functions #15914

Rationale for this change

Spark's to_binary(str[, fmt]) converts a string to binary, choosing how based
on fmt: hex, utf-8/utf8, or base64. try_to_binary is the same
function except that input it cannot convert yields NULL instead of raising an
error.

Neither is available in the datafusion-spark crate, and no core DataFusion
function can stand in for them:

• core decode accepts only base64, base64pad and hex, so there is no
  way to ask it for Spark's utf-8 conversion
• core decode always raises on input it cannot convert, so there is nothing
  to build try_to_binary's NULL-returning behaviour from

What changes are included in this PR?

• New SparkToBinary and SparkTryToBinary UDFs in
  datafusion/spark/src/function/string/to_binary.rs
  • to_binary(str[, fmt]) where fmt is a case-insensitive literal of
    hex, utf-8, utf8 or base64, defaulting to hex
  • hex decodes two characters per byte, left-padding an odd-length input
    with 0, matching Unhex
  • utf-8 / utf8 returns the string's own UTF-8 bytes
  • base64 matches Java's MIME decoder, which Spark uses: the standard
    alphabet, padding optional, and the unused trailing bits of a short final
    group ignored
  • A NULL value or a NULL fmt yields NULL in both functions
  • to_binary raises on a malformed value or an unrecognised fmt;
    try_to_binary returns NULL for both, matching nullOnInvalidFormat
  • fmt must be foldable, as Spark requires
• unhex_scalar is made pub(crate) and reused for the hex path rather
  than duplicating the decoder
• Registered in mod.rs (make_udf_function!, export_functions!,
  functions())
• Filled in the commented-out SLT scaffolds for both functions

Are these changes tested?

Yes.

• 10 unit tests in to_binary.rs (each format, hex as the default, empty
  input, NULL value and NULL format, invalid value, invalid format, column
  input, and a column where one row is invalid)
• 15 SLT assertions across spark/string/to_binary.slt and
  spark/string/try_to_binary.slt

Are there any user-facing changes?

No. These are new functions in the datafusion-spark crate only.

[arsathS]

cc @Jefffrey This is my first time contribution. Can you please take a look?