igvm: rethink api to allow dynamically adding new headers vs requiring all upfront

In order to support rolling measurements for future changes where we incorporate corim generation as part of the igvm crate, we may want to change the IgvmFile API to allow adding directive headers one-by-one, instead of requiring them all to be present initially. This would allow a user to only define platform and init headers, and then dynamically add directive headers as running through loader code, then very easily finalize the igvm file by adding a corim/measurement header via a consuming self method.

Start by locating the IgvmFile API in the igvm crate and tracing how directive headers are currently supplied. Determine how incremental additions and a consuming finalization step would support the rolling-measurement flow. Done means the intended construction and finalization behavior is specified and covered by tests.