Need proxy support in air-gapped environment

**User Story**

As an operator, I would like to add proxy setting configurations to capi providers for the air-gapped environments.

**Detailed Description**

In air-gapped environment, cluster API provider pods might be deployed in air-gapped environment, and thus cannot talk to the infrastructure provider directly. In this scenario, a proxy server is typically deployed to enable the connectivity and audit the traffic that bypasses the firewall. It would be ideal if we can have a mechanism to plumb the proxy server configurations to the cluster API provider pods, so that they can be able to communicate with the infrastructure.

**Anything else you would like to add:**
One approach I think think of is to have something like this:
```
HTTP_PROXY=xxx clusterctl init
```
The implementation should be similar to https://github.com/kubernetes/kubernetes/pull/84559.

[Miscellaneous information that will assist in solving the issue.]

/kind feature

Start at the clusterctl init entry point and review the Kubernetes pull request linked in the issue for the proposed proxy-setting approach. Determine how proxy configuration should be plumbed to Cluster API provider pods, then verify that providers can communicate through the proxy in an air-gapped environment.

Written by the indexing model from the issue text.