feat(reader): Date32 from days since epoch for Literal:try_from_json

[mbutrovich]

Which issue does this PR close?

• Partially addresses ArrowReader enhancements for Apache DataFusion Comet #1749. Just gonna copy my comments from the test:

    /// This reproduces the scenario from Iceberg Java's TestAddFilesProcedure where:
    /// - Date partition columns have initial_default values in manifests
    /// - These values are serialized as days since epoch (e.g., 18628 for 2021-01-01)
    /// - The JSON schema includes: {"type":"date","initial-default":18628}

What changes are included in this PR?

Add a match arm for deserializing a Date32 from days since the epoch.

Are these changes tested?

• New test test_date_from_json_as_number
• This fixed 6 tests in Iceberg Java's spark-extensions TestAddFilesProcedure suite when running with Comet's feat: Iceberg scan based serializing FileScanTasks to iceberg-rust datafusion-comet#2528.

[manuzhang]

This file is already 4000 lines long 🤯 ?