backend: add Transactions implementation to blockbook.

[bznein]

Note: based on #3440

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

[benma]

Please do, my ERC20 accounts don't show any transactions with this PR 😅 I should probably review once it works for ETH and ERC20, or do you want to separate it into two PRs?

[bznein]

I think since we are on a staging branch, is fine to do it in a separate PR. Definitely need to do some investigation for it :)

[benma]

This errors on my ETH account I use for testing, because some spam tx touched it:

https://etherscan.io/tx/0x1a926b5296dc6803a8a07daeebbd6b1b6b476790af5719ec628d5eeca08027f2

This tx has a ton of ERC-1155 token transfers (NFTs?), one of which touches my address.

I am not sure so you should check, but I think the Etherscan-based account includes such transactions in the list if it originated from the account, in other words, if the ETH accounts paid the fees for it (which impacts the account balance and portfolio chart calculation). I guess it does not make sense to list "incoming" token transfers in the main ETH account, these are shown separately in the respective ERC20 accounts.