feat(file_index): add indexer contracts and outer-format composition

[QuakeWang]

Purpose

Closes: #34

Bitmap and Bloom implementations already existed, but callers still had to match identifiers and construct concrete readers and writers directly. This leaked implementation details into future scan and writer integrations.

This change adds crate-private contracts and built-in dispatch so outer-format index payloads can be composed without depending on concrete index implementations.

Brief change log

• Add a common FileIndexWriter contract and built-in Bitmap/Bloom factory.
• Build per-column readers from outer-format metadata and schema.
• Handle nested map columns, empty indexes, type/options validation, and unknown identifiers.
• Add component E2E coverage for multiple columns and indexes.

Tests

• cargo fmt --all -- --check
• cargo test --locked -p paimon --all-targets --features fulltext,vortex

API and Format

Documentation

[JingsongLi]

+1