Tezlink/Kernel: Sign batches

Linear issue

Closes L2-

Next

!18917 (merged)

What

This MR changes the kernel behaviour on Tezlink so that it signs batches.

How

• To improve code readability and minimize conversions in the end, we sign the same type of values that we will apply ManagerOperation<OperationContent
• To allow modifying the code in small commits, we break the operation down when passing it downstream as branch, signature, and content. Otherwise, it'd be necessary to change the type of Operation straight away, which rapidly cascades throughout the code.
• We finally change the input type of the signature functions to a vector.

Manually testing the MR

make -f etherlink.mk test