[C++] Inconsistent handling of path-segment encoding for directory and filename partitioning #55446

The directory and filename dataset partitioning methods both accept a segment_encoding parameter, which defaults to URI encoding. However, this is only used when reading datasets, and writing ignores this option and always uses no encoding.

This means that it's impossible to write certain values that aren't valid path segments, and also means values can round-trip incorrectly if they contain values that would be interpreted as percent-encoded during reading.

This behaviour is also inconsistent with Hive partitioning, which encodes path segment values by default.

Related to #34905, which is specific to Hive partitioning and has the opposite problem, where encoding is always used when writing and can't be opted out of.

Fixing this would also solve #33507.