Support connecting to endpoints using a SOCKS5/HTTP proxy

[semekh]

Is your feature request related to a problem? Please describe.
Vault needs to have network access to different components as secret engines. This might not be feasible when:

1. The target is in a private network and does not have a public IP address
2. The target is in a network where only outbound connections are allowedE
3. One doesn't want to expose the target on a public network

Describe the solution you'd like
While providing direct network access might not be secure and maintainable, if Vault could connect to targets through a proxy server, the proxy server could do the heavy-lifting of routing, access control, audit, etc. Implementing the socks5/http proxy for handling TCP connections could be generic enough, so that the user could bring their own implementation of proxies.

Describe alternatives you've considered
The proxying could be performed through a transparent proxy mechanism, but that is less than ideal since it's all or nothing and less configurable.

Explain any additional use-cases

• The user might have an existing zero-trust network, and would like to leverage that for connecting to its endpoints. The zero-trust network might be Boundary Connect to endpoints thru Hashicorp Boundary #11459

Additional context