Add Batch Scrape Support

[0xjmp]

Implements the /v2/batch/scrape endpoint.

Changes:

• Added types: BatchScrapeParams, BatchScrapeResponse, WebhookSpec, FormatSpec, ParserSpec, ActionSpec, LocationSpec, Viewport
• Implemented BatchScrape() to start batch scrape jobs
• Added tests covering basic usage, options, webhooks, and error cases
• Improved error handling for empty responses
• Added retry logic for 503 errors
• The implementation follows the same patterns as other API methods and matches the OpenAPI specification.

$ go test -v -run TestBatchScrape                                                                                                              
=== RUN   TestBatchScrapeInvalidAPIKey
--- PASS: TestBatchScrapeInvalidAPIKey (0.34s)
=== RUN   TestBatchScrapeE2E
--- PASS: TestBatchScrapeE2E (0.16s)
=== RUN   TestBatchScrapeWithOptionsE2E
--- PASS: TestBatchScrapeWithOptionsE2E (0.10s)
=== RUN   TestBatchScrapeWithWebhookE2E
--- PASS: TestBatchScrapeWithWebhookE2E (0.35s)
=== RUN   TestBatchScrapeWithInvalidURLs
--- PASS: TestBatchScrapeWithInvalidURLs (0.09s)
=== RUN   TestBatchScrapeEmptyURLs
--- PASS: TestBatchScrapeEmptyURLs (0.00s)
=== RUN   TestBatchScrapeNilParams
--- PASS: TestBatchScrapeNilParams (0.00s)
PASS
ok      github.com/mendableai/firecrawl-go/v2   1.448s

[0xjmp]

@mogery I did notice this endpoint occasionally throw a 500 error