feat(crawler): add MHTML capture functionality

Add ability to capture web pages as MHTML format, which includes all page resources
in a single file. This enables complete page archival and offline viewing.

- Add capture_mhtml parameter to CrawlerRunConfig
- Implement MHTML capture using CDP in AsyncPlaywrightCrawlerStrategy
- Add mhtml field to CrawlResult and AsyncCrawlResponse models
- Add comprehensive tests for MHTML capture functionality
- Update documentation with MHTML capture details
- Add exclude_all_images option for better memory management

Breaking changes: None

docs/md_v2/api/parameters.md

@@ -140,6 +140,7 @@ If your page is a single-page app with repeated JS updates, set `js_only=True` i
 | **`screenshot_wait_for`**                  | `float or None`     | Extra wait time before the screenshot.                                                                    |
 | **`screenshot_height_threshold`**          | `int` (~20000)      | If the page is taller than this, alternate screenshot strategies are used.                                |
 | **`pdf`**                                  | `bool` (False)      | If `True`, returns a PDF in `result.pdf`.                                                                 |
+| **`capture_mhtml`**                        | `bool` (False)      | If `True`, captures an MHTML snapshot of the page in `result.mhtml`. MHTML includes all page resources (CSS, images, etc.) in a single file. |
 | **`image_description_min_word_threshold`** | `int` (~50)         | Minimum words for an image’s alt text or description to be considered valid.                              |
 | **`image_score_threshold`**                | `int` (~3)          | Filter out low-scoring images. The crawler scores images by relevance (size, context, etc.).              |
 | **`exclude_external_images`**              | `bool` (False)      | Exclude images from other domains.                                                                        |