Enable Firefox meek-http-helper to use an upstream proxy

The helper should be able to use an upsteam proxy, so that it can be used to implement TOR_PT_PROXY as in #8402 (moved)/proposal 232.

Commit cf81b598 (the removed part) shows how to create a per-request proxy setting.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information