New datasource: vcpkg

[JamieMagee]

This depends on the vcpkg PURL type (package-url/purl-spec#562, in progress). The ecosystem definition is tracked in ossf/osv-schema#561.

• Prepare your data - refer to the OSV Schema documentation for information on how to properly format the data so it can be accepted.
• Create a PR to reserve an ID prefix and define a new ecosystem (example). We review the records you start publishing for OSV Schema correctness and quality as part of reviewing and merging this PR.
• Prepare and publish your records via a Git repository (example). If this method isn’t ideal, we also support publishing records from REST API endpoints or through a GCS bucket(example).
• To support API querying, please create a PR to extend purl_helpers.py and create a new ecosystem in _ecosystems.py. You can refer to existing examples showing how to implement support for Semver and non-Semver ecosystems.
• Create a PR to start importing the records you are publishing into our test instance of OSV.dev and validate everything is working as intended there.
• Create a PR to start importing the records you are publishing into our production environment