refactor(eth): attach ToFilecoinMessage converter to EthCall.

[rjan90]

Reviving the work by @raulk (all credits to him) from #12062, which did not land due to a failing test and because the original function that was refactored has moved a bit.

Proposed Changes

Small refactor to clean things up:

• This converter is better placed as an EthCall method instead of a package-level function.
• The resulting package/module import graph is sane.
• Exporting this as a public method is very helpful for those who use Lotus as a library.

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title conforms with contribution conventions
• Update CHANGELOG.md or signal that this change does not need it per contribution conventions
• New features have usage guidelines and / or documentation updates in
  • Lotus Documentation
  • Discussion Tutorials
• Tests exist for new functionality or change in behavior
• CI is green