feat(submenu_export): Add export to png and pdf options for charts

[ramiroaquinoromero]

SUMMARY

In the Chart Builder and in the Dashboard, add export options for png and pdf.
If possible, give the user the option between a transparent background and a solid background (color of current theme background)
Customers are also hoping that the png will be higher quality than the jpg image

On the dashboard:

Update the current image option to specify that it's jpeg "Download as image (jpeg)"
Add "Download as image (png)" underneath the jpeg option, which opens a submenu for Transparent background and Solid background.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

after_demo.mp4

TESTING INSTRUCTIONS

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

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