Support option parameter for schema version configuration

[a5a351e7]

There is currently an issue with processing the SBOMs generated by cyclonedx-dotnet v6.1.0 in subsequent processing. The Java library cyclonedx-core-java does not yet support the new schema version v1.7; see CycloneDX/cyclonedx-core-java#791

Unless you downgrade to version v6.0.0, there is no way to generate SBOMs with a schema <v1.7.

It would be very helpful if the schema to be used could be manually specified using an additional parameter. This would make it possible to use a specific schema regardless of the version.

e.g.
-sv, --schema-version <version>

[mtsfoni]

Good news:
the readme is stale (shouldn't happen), we do have --spec-version as an argument.