feat: add optional tracing instrumentation

[LagginTimes]

Resolves #1934.

Description

This PR introduces structured tracing instrumentation to BDK’s chain‑sync crates (bdk_electrum, bdk_esplora, bdk_bitcoind_rpc, etc.) in a fully opt‑in, zero‑overhead manner.

• New feature: tracing-logs (disabled by default) that pulls in tracing = "0.1".
• Feature‑gated logging: all use tracing::{…} imports and trace!/debug! calls are wrapped in #[cfg(feature = "tracing-logs")].
• Broad coverage: logs on major operations (full_scan, sync, mempool, next_block) and key internals (batch sizes, cache hits/misses, gap‑limit stops, anchor processing).

With tracing-logs disabled, there should be no runtime impact.

Changelog notice

• Optional tracing-logs feature in bdk_electrum, bdk_esplora, bdk_bitcoind_rpc, etc., to enable tracing‑based instrumentation.

Checklists

All Submissions:

• I followed the contribution guidelines

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

[nymius]

I would avoid rust string interpolation in favor of field records. A short not interpolated message can hint you directly to the action performed, and the variables in context will give you all the remaining information.
I think this improves the discoverability of issues, and for large enterprise setups, provides a better format for log consumers to digest it (easier to produce json output)

trace!("mempool(): entering", start_height = self.start_height, last_mempool_tip = self.last_mempool_tip);

Also, these fields may be also included for the logging span of the function:

trace_span!(start_height = self.start_height, last_mempool_tip = self.last_mempool_tip)