Draft: Add basic openapi-diff script

Context

In order to check if the RPCs documentation (./docs/api/) is up-to-date, we need to be able to check differences between two documentations. It will then allow us to check the differences between the documentation stored in the commit and the generated one (generated with ./bin_openap/generate.sh).

The aim is to make this check in the CI. If there is any difference then the documentation is out of date and needs to be updated in that commit. Thus, any change in RPCs would be clearly visible in the commit changes, allowing easier review.

This merge request creates a basic script openapi_diff to check and display differences.

Manually testing the MR

• dune exec ./src/bin_openapi_diff/openapi-diff.exe ./doc/api/rpc-openapi.json ./doc/api/rpc-openapi.json should not display any change.
• Copy ./doc/api/rpc-openapi.json in a new file and make some changes. dune exec ./bin_openapi_diff/openapi_diff.exe should now display the changes you made.

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, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR