Cache assets downloaded in build.rs

[thewh1teagle]

Eg. https://github.com/juhaku/utoipa/blob/master/utoipa-swagger-ui/build.rs#L21
Example how to cache them: https://github.com/thewh1teagle/sherpa-rs/blob/main/sys/build.rs#L459

[juhaku]

I don't remember why and what was the process for the implementation to get the way it is now. But I believe that there was some cache at some point though. There was a check whether the content has already been downloaded and if it was it was not then downloaded again, this has one drawback users need to explicitly call cargo clean first so that they get the latest and greatest version.

Honestly downloading a few bytes of data every now and then does not break Github in my opinion. Another concern is to keep the build and the dependencies as small and minimal as possible. I am open for PR if there is interest in this regard, putting in effort to implement the cache in utoipa-swagger-ui as well. Currently my priorities are elsewhere.