feat(client): allow constructing without connecting

[reez]

Description

Attempts to address #166

Allows Client to be constructed without opening a network connection, adds Client::connect() for explicit connection and protocol negotiation, and defers automatic connection until the first Electrum API call.

Notes to Reviewers

I kept the patch intentionally small so I left out things like:

• Adding an eager constructor like from_config_connected for callers who want the previous construct-and-connect behavior in one step.
• Refactoring the duplicated ssl:// no-TLS check into a small helper if that feels cleaner.

Related Issues

This started for me from bdk-dart#91, where constructing an FFI backed ElectrumClient can block the Android UI thread during TCP connection and cause ANRs.

I opened bdk-ffi#1037 to address it on bdk-ffi, trying to make that blocking boundary clearer in the bindings, but I think a bdk-ffi only change still falls a bit short without an upstream change from rust-electrum-client.

So this PR is for that upstream client layer change, and is closely related to rust-electrum-client#166 where tnull asked for a way to construct an Electrum client without directly establishing a connection.