kernel/sequencer: add user space

Context

The delayed inbox, state of the sequencer kernel (and maybe other informations), should be saved under a common path in the kernel. The proposed path is __sequencer.

The kernel user should not modify or update the sequencer space. To force this behavior, the SequencerRuntime will prefix each path by /u. There are some exceptions, for instance if the path starts by /kernel/ then it will remains unchanged.

This MR introduce a function to prefix any path by /__sequencer. All read/write to the durable storage related to the kernel sequencer should use this function to prefix any path. And a function to prefix user paths by /u

Manually testing the MR

cd src/kernel_sequencer
make test
make check
make build

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