Extension of the ProjectMetadata by the properties

### Current Behavior

The ProjectMetadata model does currently not support custom properties https://github.com/DependencyTrack/dependency-track/blob/687e9970f5ae30f38014b56a4e1b9c7e6f24e80d/src/main/java/org/dependencytrack/model/ProjectMetadata.java#L46, which are present in https://github.com/CycloneDX/cyclonedx-core-java/blob/master/src/main/java/org/cyclonedx/model/Metadata.java#L93

### Proposed Behavior

Extension of the dependency track ProjectMetadata by the properties, which could be accessed and used when re-downloading the SBOM file. Additionally, the properties could be shown in the dependency track UI.

### Checklist

- [x] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/master/CONTRIBUTING.md#filing-issues)
- [x] I have checked the [existing issues](https://github.com/DependencyTrack/dependency-track/issues) for whether this enhancement was already requested