Add per-chain enabled flag to config

[anhthii]

Motivation

Currently, deploying the indexer requires passing --chains=evm,tron,... to specify which chains to index. This is inconvenient for production deployments — every deploy command or Docker entrypoint needs to maintain this list, and adding/removing a chain means updating deployment configs rather than just the application config.

Proposal

Add an enabled field to each chain's configuration in config.yaml. When --chains is not provided, only chains with enabled: true are indexed.

chains:
  tron_mainnet:
    enabled: true
    type: "tron"
    ...
  tron_testnet:
    enabled: false
    type: "tron"
    ...

Behavior:

• --chains not provided: index only chains where enabled: true (omitting the field defaults to true for backward compatibility)
• --chains provided: index the specified chains regardless of the enabled flag (preserves current behavior for manual/dev runs)

Implementation

1. Add Enabled *bool field to ChainConfig in pkg/common/config/types.go
2. Add EnabledNames() method to Chains in pkg/common/config/chains.go
3. Use EnabledNames() instead of Names() in cmd/indexer/main.go when no --chains flag is passed