[secure-storage] Support HTTPS proxy for Github backend
[secure-storage] Support HTTPS proxy for Github backend

If the `https_proxy` environment variable is set, use it with ureq for
Github requests.

Test Plan: Ran upload without `https_proxy` set, failed as before. Set
`https_proxy`, started working.

Closes: #4404
Approved by: davidiw

libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core