Regex filter on metadata value

[Emmanuel-Tyty]

Seems like a handy thing to have

• Extend TextPattern matching to recursively search through metadata arrays and maps
• Add serde(rename_all = lowercase) to MetadatumPattern for proper TOML deserialization
• Add comprehensive tests with 'testing regex' pattern
• Update metadata_regex_filter example with tested configuration and real output
• Verified on preprod testnet with successful transaction filtering

This enables filtering transactions based on metadata content patterns, making
the existing metadata value field actually usable for real-world Cardano metadata
structures which are typically nested in arrays and maps.