Embed verify.sourcify.dev to Blockscout in an iframe

[kuzdogan]

• Research options and how to make the UI work in an iframe
• New route with minimal verification form (/widget)
• Selecting the chain automatically via a parameter
• PR to Blockscout
• Update docs here: https://docs.blockscout.com/devs/verification/contracts-verification-via-sourcify

[kuzdogan]

Normally we'd need to update the legacy verification endpoints used in Blockscout to APIv2 but I guess if we ship this there won't be any need for that.

Either case we need to update the docs about Sourcify there: https://docs.blockscout.com/devs/verification/contracts-verification-via-sourcify

[manuelwedler]

PR is here: blockscout/frontend#3027