Proxy support for https vault address

[isukapalli]

Currently the gem doesn't support proxy for https connections and would like to request the support for it.

Vault::Client::SecurityError
  ----------------------------
  no direct https connection to vault

Please add the proxy support configurable for https endpoint.

[chrisarcand]

Pretty sure this was fixed in v0.18.0 (September 2023). Re-open if I'm mistaken.