Add `--allow-insecure-coinbase` to enable using a random coinbase if none is set

[ferranbt]

After PR #306, we auto-generate a random coinbase address when none is provided. While this helps with quick testing, it's a potential footgun since real coinbase addresses are required in production.

Proposed fix: Add --allow-insecure-coinbase flag that must be set to enable random address generation. This makes users explicitly opt-in to running without a proper coinbase.

[faheelsattar]

Hey! can you assign me this?

[varun-doshi]

Hi @ferranbt
I'd like to work on this if its still open