Generate the ecosystems.txt in the root

[VinodAnandan]

Could you please generate the ecosystems.txt in the root that would contain a list of all the ecosystems currently in OSV, from which users can construct URLs to download all advisories for the ecosystems?

[oliverchang]

We now have a https://osv-vulnerabilities.storage.googleapis.com/ecosystems.txt in the root.

This still needs to be generated dynamically, but our ecosystems change infrequently so it should be fine to depend on this in the short/medium term.