feat(vault): add support for Kubernetes auth method

[eufebius]

Implements authentication to Vault using the Kubernetes auth back-end.
The method is enabled via environment variables:

• VAULT_AUTH_K8S_ROLE
• VAULT_AUTH_K8S_JWT_PATH (optional, default: /var/run/secrets/kubernetes.io/serviceaccount/token)
• VAULT_AUTH_K8S_MOUNT (optional, default: "kubernetes")

Falls back gracefully if VAULT_AUTH_K8S_ROLE is not defined.

api/minor

[tombokombo]

Hi, is there any progress? Thanks

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[fcrespel]

I too would be interested in this feature (I'd need it to generate files from Vault secrets in a K8S init container), any chance of reviewing/merging it @hairyhenderson ? Thanks.

[github-actions]

This pull request is stale because it has been open for 60 days with
no activity. If it is no longer relevant or necessary, please close
it. Given no action, it will be closed in 14 days.

If it's still relevant, one of the following will remove the stale
marking:

• A maintainer can add this pull request to a milestone to indicate
  that it's been accepted and will be worked on
• A maintainer can remove the stale label
• Anyone can post an update or other comment
• Anyone with write access can push a commit to the pull request
  branch

[hairyhenderson]

Apologies for the slow response, reviewing now.

[hairyhenderson]

I've tried to rebase this but the fork repo is rejecting it (because this was issued on main).

@eufebius I'll need to ask you to rebase this first so we can get it merged.