Ability to add request headers

[via-justa]

Is your feature request related to a problem? Please describe.
We're using Cloudflare Access in front of Vault. We must pass the Cloudflare token via the cf-access-token header when accessing the vault.

When we log in to Vault directly via CLI, we use the following script.

export CF_TOKEN=$(cloudflared access login ${VAULT_ADDR} | grep -o -E ".{100,}")
export VAULT_TOKEN=$(vault login -header "cf-access-token=${CF_TOKEN}" -method=oidc $VAULT_ROLE -format=yaml | yq e ".auth.client_token" -)

Describe the solution you'd like
Ability to pass headers to the plugin when running in CLI, either by a flag or Environment variable.
Example expected usage

export VAULT_ADDR='https://vault.example.com'
export CF_TOKEN=$(cloudflared access login ${VAULT_ADDR} | grep -o -E ".{100,}")
export VAULT_TOKEN=$(vault login -header "cf-access-token=${CF_TOKEN}" -method=oidc $VAULT_ROLE -format=yaml | yq e ".auth.client_token" -)
helm template release --namespace ns my/chart | argocd-vault-plugin -H "cf-access-token=${CF_TOKEN}" generate -

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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