Add Finch HTTP client and make hackney an optional dependency

[klacointe]

Summary

Follow-up to #150. Revives the Finch client from the closed #148 (which was split up to keep that PR scoped to the adapter pattern), and completes the original goal: making :hackney an optional dependency now that a real alternative exists.

• Add Waffle.HTTPClient.Finch, guarded with Code.ensure_loaded?(Finch) — the implementation is carried over unchanged from Add pluggable HTTP client behaviour with Hackney and Finch implementations #148.
• Make :hackney optional: true. Since it remains the default configured client, Waffle.HTTPClient.Hackney is still always defined, but guarded with Code.ensure_loaded?(:hackney); if it's missing, calling it raises a clear, actionable error instead of a bare UndefinedFunctionError.
• Extract parse_content_disposition/1 onto the Waffle.HTTPClient behaviour so both clients (and any custom implementation) share one parser, instead of Hackney depending on the internal :hackney_headers module.
• Bump CI's Dockerfile to elixir:1.17-alpine (Finch requires Elixir ~> 1.15).
• Document both clients and the "at least one HTTP client dependency is required" story in the README/CHANGELOG.

Configuration

# Hackney (default) — add {:hackney, "~> 1.9"} to deps
config :waffle, :http_client, Waffle.HTTPClient.Hackney

# Finch — add {:finch, "~> 0.18"} to deps, start a pool, then:
config :waffle, :http_client, Waffle.HTTPClient.Finch
config :waffle, Waffle.HTTPClient.Finch, pool_name: MyApp.Finch

Notes

• No default behaviour change for existing users — hackney remains the default client.
• Finch has no fallback/raise stub if missing (unlike Hackney), since it's never silently defaulted to — you only reach it by explicitly configuring :http_client, so a bare UndefinedFunctionError naming the module you just typed is already self-diagnosing.

Tests

• New tests for Waffle.HTTPClient.Finch (mocked transport layer, redirects, body-size limiting, timeouts)
• New tests for the shared parse_content_disposition/1 helper
• Updated an existing store_test.exs test that relied on hackney's internal header-parsing API