Request: Etag headers for LDML API

[rmunn]

The current implementation of the LDML API relies on passing the &revid=... query parameter to avoid redownloading LDML files that haven't changed. That's pretty much exactly what the HTTP Etag header is designed for. It would nice if the LDML API server could be enhanced to return an HTTP ETag header alongside the LDML files being served up; that would allow some clients to use the If-None-Match header to skip downloading an LDML file they already have.

The expected benefit is small but non-zero: since ETag and If-None-Match are part of HTTP standard, some HTTP client libraries already incorporate them into their feature set. A project using such a library wouldn't need to care about the revid parameter, but would be able to simply use the library's existing caching feature as-is. I believe this would save programmer time in the long run.