GH-50333: [C++][Parquet] Add dense decode path for FIXED_LEN_BYTE_ARRAY

[marcin-krystianc]

Rationale for this change

For performance reasons, we would like to decode float16 (FLBA) values directly into the caller-provided buffer.

What changes are included in this PR?

• Adds a virtual int Decode(uint8_t* buffer, int max_values) to FLBADecoder in encoding.h. It writes decoded values contiguously into a caller-owned buffer.
• Implements the new overload for all supported decoders. (The base implementation throws ParquetException, so unimplemented encodings fail with a clear message.)
• Replaces the old PARQUET-1508 TODO comment.

Are these changes tested?

Yes

Are there any user-facing changes?

No

[github-actions]

Thanks for opening a pull request!

If this is not a minor PR. Could you open an issue for this pull request on GitHub? https://github.com/apache/arrow/issues/new/choose

Opening GitHub issues ahead of time contributes to the Openness of the Apache Arrow project.

Then could you also rename the pull request title in the following format?

GH-${GITHUB_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}

or

MINOR: [${COMPONENT}] ${SUMMARY}

See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview