Add a plugin API for reading HTTP response bodies

Plugins read bodies by response id through ctx.httpResponse.body()
instead of opening HttpResponse.bodyPath themselves. The accessors are
named after fetch's, minus the single-use semantics, since the bytes are
durable and re-reading should work.

Underneath is a chunked pull over the existing plugin protocol, so the
host can move bodies off the filesystem without plugins noticing.
text() now decodes with the response's charset rather than assuming
UTF-8, and the buffering accessors refuse past 32 MiB and point at
chunks().

Cargo.lock

@@ -11196,6 +11196,7 @@ name = "yaak"
 version = "0.1.0"
 dependencies = [
  "async-trait",
+ "base64 0.22.1",
  "log 0.4.29",
  "md5 0.8.0",
  "serde_json",