Etherlink+Tezlink/Kernel: blueprints contain Vec<Transaction>

What

This MR refactors the definition of blueprints in the kernel to force the content to be a vector.

Why

When adding Tezlink transactions in the kernel in !17040 (merged), we wanted runtime-specific blueprints so it was OK to let to each runtime the liberty to define the format of its blueprints. Now that we want a single Tezos X blueprint per L2 level containing an interleaving of operations from all runtimes, we need to remove this liberty to runtimes.

How

• add a Transaction (singular) type parameter to the ChainConfigTrait, which defines the interface that runtimes must implement,
• change the type definition of blueprints (in blueprint.rs) to use a Vec,
• remove the Transactions (plural) type parameter of the ChainConfigTrait,
• remove dead code.

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