Allow configuring open and read timeouts for API requests

[fluke]

Description

Fixes #1466

• Problem: HttpClient#request calls HTTParty with no timeout, so every API request uses Net::HTTP's default 60-second read timeout. When Shopify is slow to respond, the calling thread (often a web request worker) waits for the whole stall. In production we saw an Admin GraphQL read hold a Puma thread for 27.5s.
• Change: adds optional open_timeout and read_timeout (seconds) to ShopifyAPI::Context.setup. HttpClient#request passes the ones that are set to HTTParty.
• Impact: nothing changes unless an app sets them. A timed-out request raises Net::OpenTimeout / Net::ReadTimeout. The retry loop only retries 429/500 responses, so timeouts aren't retried; the README says so.

This is deliberately narrower than #1376, which exposes a general httparty_params hash. Only the two timeouts are added, as typed settings. If you'd rather take #1376, it covers this as well.

How has this been tested?

• test/clients/http_client_test.rb:
  • timeouts set on Context reach the Net::HTTP connection;
  • with none set, the connection's timeouts aren't touched;
  • a Net::ReadTimeout is raised and not retried.
• test/context_test.rb: both settings default to nil and can be configured.
• bundle exec rake test:library: 347 runs, 0 failures. rubocop and srb tc are clean.
• The first http_client_test test fails when **timeouts is removed from the HTTParty call.

Checklist:

• My commit message follow the pattern described in here
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the project documentation.
• I have added a changelog line.

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