feat: add Mosaic data file reader

[QuakeWang]

Purpose

Linked issue: #378

Paimon Rust currently does not recognize .mosaic data files, so Rust clients cannot read tables written with Mosaic by Java Paimon, PyPaimon, or paimon-mosaic-core.

This PR adds the first read-only Mosaic reader foundation behind the mosaic feature.

Brief change log

• Add optional paimon-mosaic-core dependency behind the mosaic feature.
• Add .mosaic reader dispatch when the feature is enabled.
• Implement a read-only Mosaic format reader using a memory-backed InputFile.
• Preserve projection order, empty projection, row selection, and unsupported-type errors.
• Add DataFusion feature passthrough and include mosaic in CI feature checks.

Tests

• cargo fmt --all -- --check
• cargo test -p paimon --features mosaic arrow::format::mosaic
• cargo build --features fulltext,vortex,mosaic
• cargo clippy --all-targets --workspace --features fulltext,vortex,mosaic -- -D warnings
• cargo test -p paimon --all-targets --features fulltext,vortex,mosaic

API and Format

Adds an optional mosaic Cargo feature. No writer support or storage format change is introduced.

Documentation

No user-facing documentation is added in this PR. Predicate pushdown and async random-access reading are left as follow-up work.