feat: introduce enabled flag for chains configuration and update indexing logic to use enabled chains

[vietddude]

Summary

This PR adds per-chain enablement in config so deployments no longer need to rely on --chains=... for the default startup path.

When --chains is omitted, the indexer now starts only chains that are enabled in config. When --chains is provided, the CLI still takes precedence and the specified chains are indexed regardless of the config flag.

Closes #87.

Changes

• add enabled *bool to ChainConfig
• add defaults.enabled support so chain configs can inherit a default enablement value
• add Chains.EnabledNames() to return only enabled chains
• update indexer startup to use enabled chains when no --chains flag is passed
• add config tests for:
  • omitted enabled behavior
  • inherited default behavior
  • explicit per-chain override behavior
• update configs/config.example.yaml to document the new pattern with:
  • defaults.enabled: false
  • one explicit per-chain enabled: true example

Behavior

• --chains not provided:
  • only enabled chains are started
  • if a chain omits enabled, it inherits defaults.enabled when set
  • if neither chain nor defaults sets enabled, the chain remains enabled for backward compatibility
• --chains provided:
  • the specified chains are started regardless of enabled

Testing

• env GOCACHE=/tmp/multichain-indexer-go-build-cache go test ./pkg/common/config/...