Add --header-command to add headers to each request

# Description

We want to give `anyclaude` a try but our Azure OpenAI endpoint lives behind a API gateway. To pass this we have to add the OAuth2 bearer token from logged in developer in his Coder workspace.

# Fix

Please add a command line parameter `--header-command [command]` to get headers. Adding a **--header-command** enables `anyclaude` to get everytime a fresh token from `coder external-auth access-token entra-id`.

# Expected

The `anyclaude --header-command getHeader` should return a fresh token on each request. So anyclaude is able to talk to Azure OpenAI behind the API gateway.