Add support for Composer custom-headers authentication

[retlehs]

Summary

• Adds the custom-headers authentication type introduced in Composer 2.9 (composer/composer#12372)
• Allows private repositories using custom HTTP headers for authentication
• Adds composer_authentications_using_custom_headers filter in helpers and excludes it from the other_token catch-all
• Adds tasks in both wordpress-install and deploy roles

Configuration example

# group_vars/<env>/vault.yml
vault_wordpress_sites:
  example.com:
    composer_authentications:
      - { type: custom-headers, hostname: repo.example.org, headers: ["API-TOKEN: my-api-token", "X-CUSTOM-HEADER: value"] }

Docs updates: roots/docs#577

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