feat: add one-liner wallet sync API with ElectrumSync

[ShigrafS]

Closes #1974

Description

This PR introduces a small, focused helper abstraction to reduce the boilerplate required to synchronize a wallet index with an Electrum server, addressing the usability concerns raised in #1974.

The main addition is the ElectrumSync helper and SyncOptions, which wrap existing SyncRequest and FullScanRequest flows into a single, ergonomic call while preserving the existing BdkElectrumClient cache and behavior.

Concretely, this PR:

• Adds ElectrumSync, a lightweight helper for performing Electrum syncs using a shared BdkElectrumClient
• Introduces SyncOptions to configure fast sync vs full scan, stop gap, batch size, and prevout fetching
• Exports the helper from bdk_electrum for reuse
• Adds an example demonstrating a “one-liner” full scan and fast sync using public Electrum servers

The helper is intentionally minimal and builds directly on top of existing BDK primitives without introducing a new builder API or altering wallet abstractions.

---

Notes to the reviewers

This PR is informed by the feedback and architectural concerns raised in #2059.

In particular:

• The helper preserves the BdkElectrumClient and its cache across sync calls, avoiding the performance regressions discussed previously.
• The API is intentionally small and avoids introducing a fluent/builder-style abstraction that would expand the public surface area.
• The helper does not mutate wallet state; instead, it returns the existing chain and transaction updates so that application code remains in control of how updates are applied.

The goal of this PR is to provide a convenience layer that demonstrates a simplified sync flow while remaining close to existing abstractions.
Feedback on whether this belongs as a public helper or should instead live purely as an example is very welcome.

---

Changelog notice

• Added ElectrumSync and SyncOptions helpers to simplify Electrum wallet synchronization
• Added an example demonstrating one-liner Electrum sync usage

---

Checklists

All Submissions:

• I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

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 (Provide a one liner to sync with electrum #1974)

[ShigrafS]

The PR is ready for review.
The tests are passing locally.