broadcast new transactions only over tor directly to peers

[AndySchroder]

In sparrow wallet I'd like the ability to broadcast new transactions only over tor directly to peers and skip the normally configured blockchain server. Peers should be discovered using DNS seeds and/or manually configured. TOR stream isolation should be used for connection to each peer. We should also be able to choose if we don't want to use TOR but still broadcast to a specific node (see below for motivation on this).

Motivation for this is as follows:

• You'd like to use clearnet for downloading public blockchain data that everyone gets.
• You still want to use TOR to broadcast new transactions.
• You don't want to use a centralized service such as https://blockstream.info/tx/push or https://mempool.space/tx/push .

I've created an enhanced a standalone tool that serves these needs (https://github.com/AndySchroder/bitcoin-submittx), but it would be really nice to have this feature built into sparrow wallet so that it can be used from a GUI and within Sparrow, you'd only have a single application that you would need and you don't need to deal with installing a bunch of dependencies since my tool uses python (which is uncompiled).

I'd like to be able to this feature with

• Private electrum node
• Public electrum node
• Private bitcoin core node
  • If this option is selected, we should recommend setting walletbroadcast=0.
• No bitcoin node

therefor, I don't think romanz/electrs#750 is the best way to solve this.

Also, since we we can open signed raw transactions without even having a wallet configured, we should be able to do so with this feature as well.

In addition to privacy, we may want to be able to broadcast to a different (potentially pruned and/or local) node that we own that has different fee limits and mempool size in order to workaround #1264 .