New datasource: vcpkg

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

- [ ] Prepare your data \- refer to the [OSV Schema](https://ossf.github.io/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](https://ossf.github.io/osv-schema/#id-modified-fields) ([example](https://github.com/ossf/osv-schema/pull/219)). We review the records you start publishing for OSV Schema [correctness](https://github.com/ossf/osv-schema/tree/main/validation) and [quality](https://google.github.io/osv.dev/data_quality.html) as part of reviewing and merging this PR.
- [ ] Prepare and publish your records via a Git repository ([example](https://github.com/AlmaLinux/osv-database/tree/master)). If this method isn’t ideal, we also support publishing records from [REST API endpoints](https://google.github.io/osv.dev/data/new/rest-api) or through a GCS bucket([example](https://storage.googleapis.com/android-osv/)).
- [ ] To support API querying, please create a PR to extend [purl\_helpers.py](https://github.com/google/osv.dev/blob/master/osv/purl_helpers.py) and create a new ecosystem in [\_ecosystems.py](https://github.com/google/osv.dev/blob/master/osv/ecosystems/_ecosystems.py). You can refer to existing examples showing how to implement support for [Semver](https://github.com/google/osv.dev/blob/139de7b69a2ea39e2113309b3a0a47aab920ddcf/osv/ecosystems/_ecosystems.py#L45) and [non-Semver](https://github.com/google/osv.dev/pull/3430) ecosystems.
- [ ] Create a PR to start [importing the records you are publishing into our test instance of OSV.dev](https://github.com/google/osv.dev/blob/master/source_test.yaml) and validate everything is working as intended there.
- [ ] Create a PR to start [importing the records you are publishing into our production environment](https://github.com/google/osv.dev/blob/master/source.yaml)