fix(Dashboard download): Download dashboard screenshot/PDF using SupersetClient

[Vitor-Avila]

SUMMARY

When downloading the dashboard as a screenshot/PDF via the client, a POST request is sent to /api/v1/dashboard/cache_dashboard_screenshot/, and then a following GET request has to be sent to /api/v1/dashboard/${dashboardId}/screenshot/${cacheKey}/?download_format=${format}.

This PR changes the GET request to be fired with SupersetClient as opposed to fetch, to make sure that the GUEST_TOKEN_HEADER_NAME header is included in the request (for guest users).

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

No UI changes.

TESTING INSTRUCTIONS

1. Access a dashboard via embedded.
2. Download the dashboard as PDF.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API