CLI: add new -header option to be able to add headers to all cli http request

[ylorenzati]

This PR add a new option to the vault cli: -header with can be specified multiple times. All the parameters specified like that will be added to HTTP headers for all requests.

For example, it will enable us to achieve zero trust with Cloudflare access and still be able to use vault cli

This fix #8754

[hashicorp-cla]

All committers have signed the CLA.

[ccapurso]

Hi @ylorenzati, this is an interesting concept. In addition to your changes, we will want to prevent users from providing headers with the prefix X-Vault- as these are reserved for Vault. The CLI command should fail with an error providing all the offending -header values that contain the reserved prefix. Once this change is made, would you also be able to add testing to this PR? It's prudent to prevent regressions given that this new flag would be accessible to all commands as it exists in BaseCommand.

It also appears that there are conflicts in command/base.go that will need to be resolved prior to merging.

[ylorenzati]

Hi @ccapurso,

Thank you very much for your time and your feedback. I will implement what you asked as soon as possible (Before the end of the week I hope)

[ccapurso]

Sounds good, take your time :).

[ccapurso]

Thank you for the X-Vault- check and test. Looks good!