Add pluggable HTTP client behaviour with Hackney and Finch implementations

[klacointe]

Follows up on the discussion in #147.

What changed

Extracts all network-related behaviour from Waffle.File into a pluggable
Waffle.HTTPClient behaviour, following the same pattern as Waffle.StorageBehavior.

Waffle.HTTPClient — new behaviour with a single callback:

@callback get(url :: String.t(), headers :: list(), options :: keyword()) ::
  {:ok, body()} |
  {:ok, body(), filename()} |
  {:error, :timeout | :service_unavailable | {:http_error, any()}}

A shared parse_content_disposition/1 helper is provided on the behaviour module
so custom implementations don't need to reimplement it.

Two built-in implementations:

• Waffle.HTTPClient.Hackney — default, wraps the existing hackney API
• Waffle.HTTPClient.Finch — opt-in alternative

Finch is an optional dependency. Add {:finch, "~> 0.18"} only if you want to use it. Hackney remains a regular dependency (unchanged from before this PR).

Retry logic stays in Waffle.File — it's client-agnostic and triggers on
:timeout, :recv_timeout, and :service_unavailable, so custom implementations
get retries for free.

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

Custom implementations

Any module adopting the Waffle.HTTPClient behaviour can be plugged in:

config :waffle, :http_client, MyApp.HTTPClient

Tests

• Unit tests for parse_content_disposition/1
• Unit tests for both Hackney and Finch implementations (mocked transport layer)
• Retry/dispatch integration tests in Waffle.File

[achempion]

good work 👍

left some comments, thank you for looking into this

[achempion]

Thanks for the work. PR is getting too large. Can we please move Finch into a different PR, so this one only adds the adapter pattern?

[klacointe]

Thanks for the work. PR is getting too large. Can we please move Finch into a different PR, so this one only adds the adapter pattern?

I've already spent a lot of time on this PR, I'll let you split the work if needed.