WIP: Integrated TOR proxy

[rorp]

Description

This PR adds the example code for how to use the integrated Rust Tor client to sync a BDK wallet.

It simply extends the existing blockchain examples: compact_filters_balance, electrum_backend, esplora_backend_asynchronous, and esplora_backend_synchronous.

To run an example with Tor you need to set TOR environment variable to 1 or true like:

$ TOR=1 cargo run --example=compact_filters_balance --features="compact_filters"

In this case the example code will start a Tor daemon, and configure the network client to use Tor as a SOCKS5 proxy.
compact_filters_balance also creates a hidden service for a locally running bitcoind.

Fixes #66

Notes to the reviewers

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[notmandatory]

Rather than adding TOR to the existing examples I recommend creating one self contained example that demonstrates using a TOR proxy with the esplora or electrum blockchain client.