Plugins & shell: add fee_needed_to_overtake

Context

This MR provides the fee_needed_to_overtake plugin function, which returns the minimal fee that a given operation would need to be greater than another given operation. This function will be useful to indicate the fee that an operation would need to be included in a full mempool: see !8640 (merged). This MR is about half of the implementation of #5197 (closed).

This MR depends on !6787 (merged).

Manually testing the MR

Testing the new function:

dune exec src/proto_alpha/lib_plugin/test/main.exe -- fee_needed_to_overtake

Other tests in lib_plugin have been slightly modified for engineering reasons:

dune exec src/proto_alpha/lib_plugin/test/main.exe

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• [N/A] Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• [N/A] 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