feat: add Invoices#download_link and query params on Subscriptions#get

[gastonrey]

Why

The Customer Portal needs a browser-openable link to an invoice or credit-note PDF for Hyperline-billed companies, like the download_url Chargebee returns. GET /v2/invoices/{id}/download answers with a 302 to a signed, short-lived S3 URL, but Invoices#download follows the redirect and returns the bytes. Separately, Subscriptions#get accepted no query params, so callers embedded ?include_live_billing=true in the id.

What

• Invoices#download_link(id, **params) calls GET /v2/invoices/{id}/download without following the redirect and returns a Hyperline::DownloadLink (url, expires_at).
  • expires_at is computed from X-Amz-Date + X-Amz-Expires (UTC).
  • It works for credit notes too, which are invoice documents of type refund.
  • If the API answers 200 with the PDF instead, url is nil and callers fall back to download.
  • 4xx/5xx raise the usual errors.
  • The API key is never sent to S3.
• Subscriptions#get(id, **params) sends params as the query string, e.g. include_live_billing: true. get(id) sends exactly the same request as before.

Verification

• bundle exec rspec: 150 examples, 0 failures, also under a non-UTC TZ. rubocop: no offenses.
• Read-only against the Hyperline sandbox: download_link returned the signed S3 URL, valid for 60 minutes, for an invoice and for a credit note. get(id, include_live_billing: true) returned live_billing.
• Version not bumped; entries are under ## [Unreleased] in the CHANGELOG.