Support ssh via Proxy

[9547]

Feature Request

Is your feature request related to a problem? Please describe:

In some scenarios, the TiUP deployment host may not be able to SSH directly into the machines to be deployed, but it will be able to use the SSH proxy.

Describe the feature you'd like:

Why the featue is needed:

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[9547]

/assign

[AstroProfundis]

There are also issues about http_proxy and https_proxy environment variables (#168), we need to make their differences clear to avoid users' misunderstanding.

[9547]

The SSH Proxy is only used for TiUP, and is only used for administrative control, not used for the runtime of components. Let's make this as a hidden feature right now.