Clarify blocking behavior of ElectrumClient construction

ElectrumClient::new in the generated bindings performs blocking network I/O because bdk-ffi constructs the upstream electrum_client::Client immediately.

This issue is to talk about whether bdk-ffi should document that behavior more explicitly, or expose a clearer config/connect API so downstream mobile bindings can avoid UI-thread/main-isolate misuse without diverging per language, etc.

Start at ElectrumClient::new in the generated bindings and trace where bdk-ffi constructs the upstream electrum_client::Client. Determine whether explicit blocking documentation or a config/connect API best prevents mobile UI-thread or main-isolate misuse; done means a settled approach with its downstream binding implications recorded.

Written by the indexing model from the issue text.