fix: custom ecosystem work and custom network fixes

[antazoey]

What I did

The custom networks for new ecosystems kinda made no sense, so this PR fixes that.

Before:

networks:
  custom:
    - name: shibarium-mainnet
      ecosystem: polygon
      chain_id: 109
      
geth:
  polygon:
    shibarium-mainnet:
      uri: https://www.shibrpc.com

used via --network polygon:shibarium-mainnet:geth which makes less sense.

After:

networks:
  custom:
    - name: mainnet
      ecosystem: shibarium
      base_ecosystem_plugin: polygon  # Closest base class.
      chain_id: 109

geth:
  shibarium:
    mainnet:
      uri: https://www.shibrpc.com

used via --network shibarium:mainnet:geth which makes more sense.

How I did it

• Change ecosystem to be allowed to be any str and advise users to put custom names here when it makes sense
• Add new config option base_ecosystem_plugin to refer to the base-class to use.
• Fix issues where custom networks had the wrong networks
• Fix issues with default provider in custom networks.

How to verify it

Checklist

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

[fubuloubu]

Some thoughts, looks good otherwise