Add support for SSH over HTTPS tunneling

[pjbgf]

Some clusters have egress traffic rules by which all traffic must go through specific proxies.
As of v0.32.0, Flux supports:

• HTTP(S) proxies when accessing Git servers via HTTP(S).
• SOCKS5 proxies when accessing Git servers via SSH.

An additional use case is when users need to establish SSH connections whilst only having available HTTPS proxies.
SSH over HTTPS tunneling support is required for this to work.

References:

• Use case reported in the CNCF chat.
• More details on current support:
  • https://fluxcd.io/docs/cheatsheets/bootstrap/#using-https-proxy-for-egress-traffic
  • https://fluxcd.io/docs/cheatsheets/bootstrap/#git-repository-access-via-socks5-ssh-proxy