Use CodesHandle and refactored CodesContent in fdb-patch

[pgeier]

Description

This PR is related to https://github.com/ecmwf/metkit/pull/128 - it uses the new CodesHandle directly instead of eccodes C API.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

🌈🌦️📖🚧 Documentation Z3FDB 🚧📖🌦️🌈 https://sites.ecmwf.int/docs/dev-section/z3fdb/pull-requests/PR-198

🌈🌦️📖🚧 Documentation FDB 🚧📖🌦️🌈 https://sites.ecmwf.int/docs/dev-section/fdb/pull-requests/PR-198