Add a DeFi dropdown and pass custom query params to the iframe

[maxaleks]

Description and Related Issue(s)

Resolves https://github.com/blockscout/frontend/issues/1889

Adds a new env var: NEXT_PUBLIC_PAYMENT_LINK_URL

Checklist for PR author

• [x] I have tested these changes locally.
• [x] 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.
• [x] If I have added, changed, renamed, or removed an environment variable, I have updated the list of environment variables in the documentation and made the necessary changes to the validator script according to the guide