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().

crates/yaak/Cargo.toml

@@ -6,6 +6,7 @@ publish = false
 
 [dependencies]
 async-trait = "0.1"
+base64 = "0.22.1" # For carrying body chunks over a text-only plugin transport
 log = { workspace = true }
 md5 = "0.8.0"
 serde_json = { workspace = true }