Update 'version' to be required field

[justahero]

According to the JSON specification (in 1.3 & 1.4) the top-level field 'version' is required, it defaults to 1.

Therefore the field is refactored to not be optional.

• update conversion & serialization functions

[Shnatsel]

Now that I am actually using this in cargo auditable, it seems that this is not a good idea. Making it required results in it always being written to the BOM, which prevents clean roundtrips. And version: 1 is redundant because the spec states it should be treated as 1 if not present, and takes up valuable space in the cargo auditable use case.

I would like to revert this in the next semver-breaking release, to give the caller more control over the serialized SBOM and enable cleaner roundtrips with fewer alterations. I would also like to provide an accessor function instead that returns the version if set or 1 if not in the next release, to get easy access to the spec-compliant logic.