Add Steward to the CycloneDX specficiation

## Add Steward to the CycloneDX Specification

This is a suggestion to add a field in the specification to indicate if there is a steward (see, EU-CRA - Article 24 and https://linuxfoundation.eu/cyber-resilience-act for context) for the project. Ultimately, collection of this field (especially for automted scanners) may depend on an ecosystem adoption of a steward.md file within a repo so this field can be easily identified. There is a parallel issue filed with the SPDX standard at https://github.com/spdx/spdx-3-model/issues/855.

Thank you!