Update Makefile: add disperse-test-blob

This'll make it easier to test that the da-proxy is working manually. eg:
```
make run-memstore-server
make disperse-test-blob
```

Makefile

@@ -34,6 +34,9 @@ stop-minio:
 run-memstore-server:
 	./bin/eigenda-proxy --memstore.enabled
 
+disperse-test-blob:
+	curl -X POST -d my-blob-content http://127.0.0.1:3100/put/ --output stdout
+
 clean:
 	rm bin/eigenda-proxy