Scoru,Node: optionally use boot sector from a file

Context

Closes #5481 (closed)

We want to introduce the feature of bootstrap smart rollups in the protocol (to facilitate the autoamtic deployements of kernels in testnets). I've already started to work on the protocol part here !8552 (merged). But it will not work with the current node, because the node use the smart rollup originate operation to retrieve the correct boot sector. Whereas if the rollup is bootstrapped, this will be missing and the rollup will not be able to run.

In this merge request I'm proposing a new argument boot_sector_file that specifies a path to the boot sector. If the argument is not provided it will use the current mechanism to retrieve the boot sector. I'm not sure this should be recommended to use: this is unsafe, if you use the wrong boot sector you'll don't manage to produce valid commitments? (or even worse publish wrong ones).

One alternative would be to rely on the boot sector file only if the current get_boot_sector fail (and I believe the main reason of error would be that the rollup was bootstrapped).

Manually testing the MR

I will add integration tests if this new flag is accepted.

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