Proto/AI: make the type of the launch_cycle context field optional

Context

Until now we gave a special meaning to the absence of the adaptive_inflation_launch_cycle context key. This sometimes lead to several read attempts of the key during the application of a single block, which the proxy server does not appreciate.

This MR changes the type of the key to Cycle.t option which means that we are more explicit about the special None value.

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