[Do Not Review yet] backend: add SendTransaction implementation for blockbook.

[bznein]

Note: based on #3440, review that first

Before asking for reviews, here is a check list of the most common things you might need to consider:

• updating the Changelog
• writing unit tests
• checking if your changes affect other coins or tokens in unintended ways
• testing on multiple environments (Qt, Android, ...)
• having an AI review your changes

[bznein]

Note: not ready for review, I need blockbook on testnet first