feat: Add enable-p2p flag to fuel-core CLI

[bvrooman]

Related issues:

• Add instruction of how to debug the fuel-core #1236

This PR disables P2P by default, and adds a new flag to the fuel-core CLI to allow users to expressly enable P2P:

--enable-p2p

Currently, the P2P service is only enabled when the user provides CLI arguments for keypair and network. If either of these values are missing, P2P is implicitly disabled without warning to the user. There is no way to tell if the user accidentally omitted these values, or deliberately omitted them in order to disable P2P.

A better user experience is to enable P2P explicitly using --enable-p2p and to make keypair and network mandatory. This will raise an error if either of these values is missing. P2P should be disabled by default. This flag replaces having to add or remove the network argument to enable or disable P2P implicitly.

If a user wants to run fuel-core with P2P, and supplies --enable-p2p, but forgets to include keypair and/or network, they will encounter an error:

error: the following required arguments were not provided:
  --keypair <KEYPAIR>
  --network <NETWORK>

[bvrooman]

As discussed in the sync today, we will change the approach to have P2P disabled by default, and instead use a flag to enable it. I will rework this PR with that feedback. In the meantime, I am putting it back in draft.