EVM Node: Add support for --network to download kernel

What

The download kernel command has been updated to accept the --network argument. This enhancement enables the command to utilize the default, per-network preimages endpoint configuration when the user does not explicitly supply the --preimages-endpoint flag.

Why

This change improves the user experience by removing the need to manually look up and specify preimage endpoint URLs for standard networks. By simply specifying the network (e.g., mainnet, testnet), the tool can automatically fall back to the correct infrastructure settings, reducing the likelihood of configuration errors.

How

The argument parsing logic in etherlink/bin_node/main.ml is updated to include supported_network_arg within the preemptive_kernel_download_command. The retrieved network parameter is then propagated to the configuration setup. Additionally, the changelog is updated to document this new CLI feature, and the regression test expectations for the command's manual page are adjusted to reflect the presence of the new flag.

Manually testing the MR

octez-evm-node download kernel farfadet --data-dir foobar --network mainnet

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR