JSON Serialization: Add ability to include $schema property

For the CycloneDX JSON boms i generate with the library, i would find it convenient to add the JSON schema URL (top-level $schema property) to them upon serialization.
It aids in manual inspection of the bom during development.

I did not find a way to add it (apart from manually post-processing the serialized file), but maybe i was just too stupid to find it.

The issue does not name a file or test. Start by locating the JSON serialization entry point and existing CycloneDX JSON output tests, then determine how the top-level $schema value can be included during serialization. Done means serialized CycloneDX JSON can contain the schema URL without requiring manual post-processing.

Written by the indexing model from the issue text.