feat: resolve per-extractor chain against custom chains

[brunoguerios]

Implements items 3 & 4 of ENG-5744. Stacked on #909

The ENG-6199 registry refactor lets --chains resolve a custom chain name
against the extractors file's chains: section, but the per-extractor
chain: field was still parsed straight into Chain by serde. Since
Chain::Custom is externally tagged ({custom: name}), a bare
chain: tempo failed to parse, and the registry-aware Chain::from_str
didn't exist yet at parse time. So an operator could name a custom chain
on the command line but not on an individual extractor.

Changes

• ExtractorConfig now reads the YAML chain: key into a plain
  chain_name: String and keeps chain: Chain as a skipped, default
  field resolved after parse. The five downstream self.config.chain
  reads and ExtractorConfig::new's signature are unchanged;
  new sets chain_name from the passed Chain's Display.
• ExtractorConfigs::resolve_extractor_chains resolves every extractor's
  chain_name against the ChainConfigRegistry (reusing resolve_chain),
  called right after the registry is built and before the config is moved
  into task creation. An unknown name fails fast with a setup error.
• New crates/tycho-indexer/extractors.example.yaml template documenting
  every per-extractor field plus the optional top-level chains: section,
  including a chain: tempo entry that exercises the custom-chain flow.

Production crates/tycho-indexer/extractors.yaml (mainnet-Ethereum) is
untouched. Operators point the indexer at their own extractors file
carrying the matching chains: entry.

Testing

• New unit test resolves a config with chain: tempo and chain: ethereum
  against a registry built from the chains: section, asserting
  Chain::custom("tempo") / Chain::Ethereum, and that an unknown name
  returns ExtractionError::Setup.
• cargo test -p tycho-indexer, clippy, and /run-ci pass.