Add FreeBSD datasource

FreeBSD is in path to convert current vulnerability database which is in VuXML-format to OSV format. There is currently only testing repository available but there is decision for adding FreeBSD things to OSV Schema at the beging and rest when they are ready.

- [x] 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.

- [x] 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)