Moved common HTTP polling helpers into feedkit and implemented support for ETag and Last-Modified

sources/doc.go

@@ -11,4 +11,14 @@
 //
 // A single source may emit 0..N events per poll or stream iteration, and those
 // events may span multiple event kinds.
+//
+// HTTP-backed polling sources can share NewHTTPSource for generic HTTP config
+// parsing and conditional GET behavior. The helper understands:
+//   - params.url
+//   - params.user_agent
+//   - params.conditional (optional, default true)
+//
+// When validators are available, NewHTTPSource prefers ETag/If-None-Match and
+// falls back to Last-Modified/If-Modified-Since. A 304 Not Modified response is
+// treated as a successful unchanged poll.
 package sources