Add custom module for fil pubsub

[elmattic]

Summary of changes

Changes introduced in this pull request:

• Add partial support for Filecoin.ChainNotify method

Note: To fully support the endpoint, the type "revert" needs to be implemented (we currently support "current" and "apply").

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation,
• 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.