Add API for Static SDKs

[0xTim]

We should provide a JSON API for providing the URLs and checksums for the static SDKs so it's easy to work out where to get the static SDK for a specific Swift version. This will make it easier to build tools like swiftlang/github-workflows#40 and https://swift-open-source.slack.com/archives/C04MQHWCQKW/p1729591270536819

[timsneath]

Sounds like we might also need to be able to query the matching Python version (3.9? 3.10?) for a given build.