feature: allow exporting vault token from auth methods

[oulman]

Feature

Add a configuration option to auth {} blocks like JWT or AppRole which enables exporting the fetched token as a VAULT_TOKEN environment variable.

Add a documentation call out that CI should mask VAULT_TOKEN as a protected variable and NOT display it in CI/CD jobs or other logs.

Example

auth "gitlab" {
    method = "jwt"
    path = "gitlab"
    priority = 100

+  export_vault_token = true
  
<snip>
}

[oulman]

added in v0.5.0