volume: gate FetchAndWriteNeedle behind admin auth and refuse internal endpoints (#9441)

volume: require admin auth and refuse loopback endpoints in FetchAndWriteNeedle

Gate the RPC behind checkGrpcAdminAuth for parity with the rest of the
destructive volume-server RPCs, and reject cluster-internal remote S3
endpoints (loopback / link-local / IMDS / RFC 1918 / CGNAT) before
dialing. Pin the validated address against DNS rebinding by routing the
AWS SDK through an HTTP transport whose DialContext re-resolves the host
and re-applies the deny list on every dial, so an endpoint that resolves
to a public IP at validate-time and then flips to 127.0.0.1 at connect
time is refused. Operators that legitimately fetch from private hosts
can opt out with -volume.allowUntrustedRemoteEndpoints.

test/s3/remote_cache/Makefile

@@ -127,6 +127,7 @@ start-primary: check-deps
 		-webdav.port=$(PRIMARY_WEBDAV_PORT) \
 		-s3.allowDeleteBucketNotEmpty=true \
 		-s3.config=s3_config.json \
+		-volume.allowUntrustedRemoteEndpoints \
 		-dir=$(PRIMARY_DIR) \
 		-ip=127.0.0.1 \
 		-ip.bind=127.0.0.1 \