Patch to support VAULT_HTTP_PROXY variable

[mikeyyuen]

What has been changed

This patch creates VAULT_HTTP_PROXY which can be used to override the default behavior of the Vault API Client.

• Without VAULT_HTTP_PROXY set the Vault API Client will continue to work as before using the default Go HTTP behavior, reading HTTPS_PROXY as before so the changes should likely not affect any existing users.
• With VAULT_HTTP_PROXY set, the Vault API Client creates a specific Go http transport with the proxy set via the Go API forcing it to use the specified proxy to communicate with the Vault binary.

Why is this useful

If the Vault Client has been embedded in software (e.g. terraform provider, consul-template or other) that might need to use different proxies to talk to vault vs other upstream services such as AWS, Azure, GCP (as described in hashicorp/terraform-provider-vault#1150), this allows the vault behavior to be specifically controlled.

[hashicorp-cla]

All committers have signed the CLA.

[hsimon-hashicorp]

Hello, and welcome to the HashiCorp Vault community! Please note that we are moving to a process where PRs need to be associated with an issue, either an existing one or a new issue, so that the community and the engineers can discuss implementation details, strategy, and where the request fits into an existing roadmap. Additionally, all PRs do need to have the CLA signed. Please feel free to create or link an existing issue to this PR. Thanks!

[davidducros]

The CLA signing has now been recognised properly and an issue to cover this has been created here: #12613

[ncabatoff]

Changes look good, could you also update https://github.com/hashicorp/vault/blob/main/website/content/docs/commands/index.mdx#environment-variables please?

[ncabatoff]

Oh, and we'll also need a changelog entry, as described in https://github.com/hashicorp/vault/blob/main/CONTRIBUTING.md#changelog-entries

[ncabatoff]

Looks good, thanks! You'll need to merge in the latest changes from main so the tests pass before I can merge this.