How to store secrets longer than 4096 chars?

## Prerequisites

* [x] I am running the latest version. (`up upgrade`)
* [x] I searched to see if the issue already exists.
* [x] I inspected the verbose debug output with the `-v, --verbose` flag.
* [x] Are you an Up Pro subscriber?

## Description

I am trying to provide certificates to my service, but up's method for storing secrets (encrypted environment variables) doesn't support strings of this length (> 4096 chars). How should I store lengthy secrets?