manifest/product_octez: move context_related libs

What

This MR is a first step toward removing direct access to context and context_backends sublibs.

The main goal is to enforce the usage of context_ops wich is a wrapper around the mutliple context backends.

Why

To ensure all components behave consistently with the selected backend. Currently someone could start a binary with Brassaia but lib_context or Irmin is directly used in the code and thus trigger usage of Irmin in some places.

How

By removing access to lib_context and lib_context_brassaia.

A future effort could try to remove direct access to Irmin/Brassaia backends but this demands to expose their common interface in the wrapper which is not possible currently.

Manually testing the MR

The MR does not change anything beside build and cleaning previous misusage of lib_context in octez. The CI should sun correctly.

Other than that you can try to build octez after changing some calls to Context_ops.whatever with a call to Context.whatever and observe compilation errors 😄

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