Kernel/SDK: Added queue_raw_buffer to the outbox queue

What

Added queue_raw_buffer to the OutboxQueue which is used by the Bridge to store serialized/encoded messages.

Why

This function is required to add arbitrary encoded messages to the outbox queue when they are created as a TransactionResult during bridge precompile execution. See usage here.

For the context, here is the whole FA bridge diff in one big MR

How

This MR simply moves part of the already implemented queue_message logic and moves it to a separate function. Also, there is a very simple test added.

There shouldn't be any side effects, as far as I understand.

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