Implement IPFS in the end to end tests

[jpraynaud]

Why

The ipfs-devnet provides a private local IPFS network (#3434), but it is not wired to the Mithril end to end tests yet. Without this connection, the IPFS artifact distribution path cannot be exercised in a reproducible environment.

What

Add a configuration option to the mithril-end-to-end test runner that activates IPFS, and connect the IPFS devnet nodes to the Mithril nodes of the test infrastructure.

How

• Add a --use-ipfs (unstable?) option to the mithril-end-to-end CLI and propagate it to the infrastructure configuration
• Bootstrap and start the ipfs-devnet swarm when the option is activated
• Propagate the IPFS node endpoints to the aggregators and the clients
• Stop the swarm and collect the IPFS node logs when the test completes
• Document the new option in the end to end tests README