Octez: Add lib_context_brassaia

What

To allow for safe and simple selection of Brassaia or Irmin, we introduce lib_context_brassaia that is a fork/copy of lib_context using Brassaia as a backend instead of Irmin.

Why

To be able to safely test and observe Brassaia behaviours before putting it as a default for octez.

How

Manually testing the MR

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