core/file: add API for the `oC` command

Your checklist for this pull request

[x] I've read the guidelines for contributing to this repository
[x] I made sure to follow the project's coding style
[x] I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
[x] I've added tests that prove my fix is effective or that my feature works (if possible)
[ ] I've updated the rizin book with the relevant information (if needed)

Detailed description

Added missing API corresponding to the oC command, which is used often in RzIL setups.

Test plan

CI is green

Closing issues

rizinorg / rizin