Specify headers by environment variable

[JacobHenner]

Allow vault CLI HTTP headers to be specified by environment variable. This is especially useful in situations where header values should not be exposed as vault command parameters (secrets, for example). This builds on the work introduced in #8754.

Fixes #16663

[hashicorp-cla]

All committers have signed the CLA.

[VioletHynes]

Hey there @JacobHenner ! Sorry that this has lingered for a while, but this looks great.

As well as the comment I left about the conversion, there's a few other things I'd like to see before we approve and merge this:

• The name of the environment variable should be in the changelog!
• An entry in website/content/docs/commands/index.mdx (our docs for the CLI) for the header and what it does (plus an example!)
• A test of the behaviour, to make sure it behaves as expected. For prior art, CLI: add new -header option to be able to add headers to all cli http request #12508 might be a good starting point

If all of that is addressed, I'd be happy to get this merged. Let me know if you have any questions, and thank you for your patience in getting eyes on this.

[JacobHenner]

Hey there @JacobHenner ! Sorry that this has lingered for a while, but this looks great.

As well as the comment I left about the conversion, there's a few other things I'd like to see before we approve and merge this:

* The name of the environment variable should be in the changelog!

* An entry in `website/content/docs/commands/index.mdx` (our docs for the CLI) for the header and what it does (plus an example!)

* A test of the behaviour, to make sure it behaves as expected. For prior art, [CLI: add new -header option to be able to add headers to all cli http request #12508](https://github.com/hashicorp/vault/pull/12508) might be a good starting point

If all of that is addressed, I'd be happy to get this merged. Let me know if you have any questions, and thank you for your patience in getting eyes on this.

Done

[VioletHynes]

Hey there! Thanks for the updates, this looks great. I'm running all of the tests but assuming everything looks good I'll try and get this merged today.

We really appreciate your patience in getting this seem to. This is a fantastic value add, so I'm really excited to get it in.

[VioletHynes]

@JacobHenner One last thing before I can merge this -- this is failing our linter, could you run make fmt and push that commit? Once that's added, I can merge this! Everything else is passing!

Thanks in advance :)

[JacobHenner]

@JacobHenner One last thing before I can merge this -- this is failing our linter, could you run make fmt and push that commit? Once that's added, I can merge this! Everything else is passing!

Done, sorry for not catching earlier.

[VioletHynes]

Merged! Thanks again for this contribution, and we really appreciate your patience in getting this seen to and merged.

🚢 🚢 🚢