Embed verify.sourcify.dev as Sourcify verification method

[manuelwedler]

Description and Related Issue(s)

This embeds the Sourcify UI (https://verify.sourcify.dev) in an iframe when Sourcify is chosen as the contract verification method. This gives the option to use all kinds of verification methods (std json, metadata, single file) on Sourcify for Solidity and Vyper contracts.

The old Sourcify verification method was outdated. It used an deprecated endpoint and didn't allow for Vyper verification.

Proposed Changes

This simply embeds verify.sourcify.dev in an iframe in the ContractVerificationSourcify.tsx component.

I wasn't sure if the verify.sourcify.dev URL should be an environment variable, because it shouldn't be changed across Blockscout instances and stay the same everywhere. Let me know if you still want it to be an env variable and I'll make the change.

Breaking or Incompatible Changes

None

Checklist for PR author

• I have tested these changes locally.
• I added tests to cover any new functionality, following this guide
• Whenever I fix a bug, I include a regression test to ensure that the bug does not reappear silently.
• If I have added, changed, renamed, or removed an environment variable
  • I updated the list of environment variables in the documentation
  • I made the necessary changes to the validator script according to the guide
  • I added "ENVs" label to this pull request