Report the interface ID used to capture pcapng files

[emarch06]

• On pcapng files, you can have several capture interfaces. We report this information to be able to use it on another classes
  I don't think adding a unit test for this is worth it, since We just report a field to the upper layer that is already parsed.

Note: It's my first contribution to scapy. Let me know if I missed something :-)

Checklist:

• If you are new to Scapy: I have checked CONTRIBUTING.md (esp. section submitting-pull-requests)
• I squashed commits belonging together
• I added unit tests or explained why they are not relevant
• I executed the regression tests (using cd test && ./run_tests or tox)
• If the PR is still not finished, please create a Draft Pull Request

fixes #4731