feat!: Make HTTP client pluggable with abstract base classes (#641)

## Summary

- Introduce `HttpClient` and `HttpClientAsync` ABCs that users can
extend to provide custom HTTP client implementations via
`ApifyClient(http_client=...)` and `ApifyClientAsync(http_client=...)`
- Factor impit-specific logic into
`ImpitHttpClient`/`ImpitHttpClientAsync`, keeping `_base.py` free of any
`impit` dependency
- Add `HttpResponse` protocol with full streaming support (`iter_bytes`,
`aiter_bytes`, `read`, `aread`, `close`, `aclose`)
- Export all public types (`HttpClient`, `HttpClientAsync`,
`HttpResponse`, `ImpitHttpClient`, `ImpitHttpClientAsync`) from
`apify_client`

## Issue

- Closes: #416

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: vdusek

scripts/check_async_docstrings.py

@@ -11,6 +11,12 @@
 
 found_issues = False
 
+# Methods where the async docstring is intentionally different from the sync one
+# (e.g. because they accept different parameter types).
+SKIPPED_METHODS = {
+    'with_custom_http_client',
+}
+
 # Get the directory of the source files
 clients_path = Path(__file__).parent.resolve() / '../src/apify_client'
 
@@ -37,6 +43,10 @@
             if len(async_method.decorators) and str(async_method.decorators[0].value) == 'ignore_docs':
                 continue
 
+            # Skip methods whose docstrings are intentionally different
+            if async_method.name in SKIPPED_METHODS:
+                continue
+
             # If the sync method has a docstring, check if it matches the async dostring
             if sync_method and isinstance(sync_method.value[0].value, str):
                 sync_docstring = sync_method.value[0].value

scripts/fix_async_docstrings.py

@@ -6,6 +6,12 @@
 from redbaron import RedBaron
 from utils import sync_to_async_docstring
 
+# Methods where the async docstring is intentionally different from the sync one
+# (e.g. because they accept different parameter types).
+SKIPPED_METHODS = {
+    'with_custom_http_client',
+}
+
 # Get the directory of the source files
 clients_path = Path(__file__).parent.resolve() / '../src/apify_client'
 
@@ -34,6 +40,10 @@
             if len(async_method.decorators) and str(async_method.decorators[0].value) == 'ignore_docs':
                 continue
 
+            # Skip methods whose docstrings are intentionally different
+            if async_method.name in SKIPPED_METHODS:
+                continue
+
             # Skip methods that don't exist in the sync class
             if sync_method is None:
                 continue

src/apify_client/__init__.py

@@ -1,7 +1,23 @@
 from importlib import metadata
 
 from ._apify_client import ApifyClient, ApifyClientAsync
+from ._http_clients import (
+    HttpClient,
+    HttpClientAsync,
+    HttpResponse,
+    ImpitHttpClient,
+    ImpitHttpClientAsync,
+)
 
 __version__ = metadata.version('apify-client')
 
-__all__ = ['ApifyClient', 'ApifyClientAsync', '__version__']
+__all__ = [
+    'ApifyClient',
+    'ApifyClientAsync',
+    'HttpClient',
+    'HttpClientAsync',
+    'HttpResponse',
+    'ImpitHttpClient',
+    'ImpitHttpClientAsync',
+    '__version__',
+]