Add background indexer

[elmattic]

Summary of changes

Changes introduced in this pull request:

• Make the Eth mappings indexer run in the background
• Disable indexing by default
• Add new sections to the configuration:

  [fevm]
  enable_eth_rpc = false
  eth_trace_filter_max_results = 500

  [chain_indexer]

• Add FOREST_FEVM_ENABLE_ETH_RPC environment variable
• Remove eth_mapping_ttl from [client] config section

Reference issue to close (if applicable)

Closes #4505

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation. All new code adheres to the team's documentation standards,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

[elmattic]

Note that the initial step of populating the index is still always performed. In a future PR, we aim to remove it entirely and provide the user full control of this process via a subcommand:

forest-tool index validate-backfill --from<epoch> --to <epoch>

This approach is similar to the one used by Lotus.

[lemmih]

I like it. It'll make my chain muxer PR a little bit simpler.