feat: allow forking custom networks automatically

[antazoey]

What I did

allows forking custom networks automatically

fixes: #1905

NOTE: Works best with this one #2144

How I did it

if the provider is known for forking, just send it on through anyway.
had to do some finaggling throughout

How to verify it

my cfg:

networks:
  custom:
    - name: mainnet
      chain_id: 109
      ecosystem: shibarium
      base_ecosystem_plugin: polygon
      default_provider: node

Note: cfg is small because using evmchains via #2144

Checklist

• All changes are completed
• New test cases have been added
• Documentation has been updated