Add Customizable HTTP timeouts

[wpeterson]

This PR adds the ability to customize the HTTP timeouts used when fetching content for prerendering via Net::HTTP:

• timeout for both read_timeout and open_timeout
• read_timeout
• open_timeout

---

Note

Adds configurable Net::HTTP timeouts and refactors HTTP request/client creation with accompanying tests.

• Prerender middleware (lib/prerender_rails.rb):
  • Add configurable timeouts: supports global timeout, and per-option open_timeout and read_timeout (applied to Net::HTTP).
  • Refactor HTTP calls:
    • New build_http_client(uri) sets use_ssl, open_timeout, and read_timeout.
    • New build_http_request(uri, headers) builds Net::HTTP::Get and applies basic auth when enabled.
    • get_prerendered_page_response now uses these helpers.
• Tests (test/lib/prerender_rails.rb):
  • Add specs for build_http_client covering SSL selection and timeout configuration.
  • Keep existing URL-building tests intact.

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[wpeterson]

@thoop Would love if you had time to take a look at this PR. Being able to customize timeouts seem like a key safety mechanism.