bpf2c_plugin doesn't support --elf argument

### Describe the feature you'd like supported

The bpf_conformance_runner.exe passes --elf to the plugin if --elf is specified as an argument to bpf_conformance_runner.
However the plugin has no such option and reports an error.

### Proposed solution

Implement the argument

### Additional context

_No response_

Start by tracing how bpf_conformance_runner.exe forwards --elf and how bpf2c_plugin parses its arguments. Verify the plugin accepts the option without reporting an unknown-argument error, and run the relevant conformance workflow to confirm the forwarded ELF path is handled.

Written by the indexing model from the issue text.