FM: Implement SubnetManager and integrations to Fendermint

[adlrocha]

The agent currently support the connection with Lotus FVM and FEVM-based subnets. The goal of this issue is to implement the integration of the agent with an underlying FEVM Fendermint subnet.

This implied the implementation of:

• The SubnetManager and all the end-to-end operations for the lifecycle of a subnet.
• The checkpoint manager.
• Any other command implemented through the CLI.

Under the hood we will be using the ethers library to interact with Fendermint, so I would expect this integration to be "fairly simple"

[raulk]

@adlrocha can you update this issue with the current status?