optimize PLAIN string columns in parquet to dictionary array #10590

Is your feature request related to a problem or challenge?

When reading a PLAIN-encoded string/binary column into a Dictionary Arrow type, the current sad path in ByteArrayDictionaryReader materializes string data twice:

1. Page bytes -> OffsetBuffer (copy #/1)
2. OffsetBuffer -> StringArray -> GenericByteDictionaryBuilder which hashes every value and copies unique ones into the dict values buffer (copy #/2)

each row also incurs a hash cost. Not sure if this is avoidable but worth flagging

Describe the solution you'd like

1. add benchmarks and validate that the existing ones cover the changes this issues intends to make

2. Do a single-pass decode that maintains the dedup HashTable during page scanning. Since PLAIN byte array encoding is [4-byte length][bytes], the bytes are already in memory when we scan them. We can hash before copying, check the table, and only write new entries into the dictionary values buffer — emitting key indices directly.

Describe alternatives you've considered

n/a

Additional context

• Epic: Cost-Based Dictionary Encoding datafusion#24111
• follow up flagged by @/alamb Epic: Cost-Based Dictionary Encoding datafusion#24111 (comment)
• Allow users to enable dictionary column reads from parquet files datafusion#24112 focuses on RLE encoded parquet columns, this PR would focus on speeding up reads for PLAIN encoded strings to dictionary arrays