Introduce structured, span-based observability through Logger interface

[wvanlint]

This change allows users to create hierarchical span objects through the Logger interface for specific computations, such as the handling of HTLCs. These span objects will be held in LDK across the corresponding lifetimes before being dropped, providing insight in durations and latencies.

This API is designed to be compatible with https://docs.rs/opentelemetry/latest/opentelemetry/trace/trait.Tracer.html, but can also be used more directly to derive time-based metrics or to log durations.

Hierarchical RAII spans are currently added for:

• HTLC lifetimes, HTLC state transitions, inbound to outbound HTLC forwarding (see functional_tests.rs).
• Ping/pong request-response pairs.

[ldk-reviews-bot]

👋 Hi! I see this is a draft PR.
I'll wait to assign reviewers until you mark it as ready for review.
Just convert it out of draft status when you're ready for review!

[codecov]

Codecov Report

Attention: Patch coverage is 84.86395% with 89 lines in your changes missing coverage. Please review.

Project coverage is 89.37%. Comparing base (89f5217) to head (40a6022).

Files with missing lines	Patch %	Lines
lightning/src/ln/channel.rs	84.84%	32 Missing and 28 partials ⚠️
lightning/src/ln/channelmanager.rs	76.36%	13 Missing ⚠️
lightning/src/util/logger.rs	73.91%	6 Missing ⚠️
lightning/src/chain/channelmonitor.rs	0.00%	3 Missing ⚠️
lightning/src/ln/invoice_utils.rs	0.00%	3 Missing ⚠️
lightning-dns-resolver/src/lib.rs	0.00%	1 Missing ⚠️
lightning-net-tokio/src/lib.rs	0.00%	1 Missing ⚠️
lightning/src/ln/onion_payment.rs	83.33%	0 Missing and 1 partial ⚠️
lightning/src/util/test_utils.rs	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3766      +/-   ##
==========================================
+ Coverage   89.35%   89.37%   +0.01%     
==========================================
  Files         157      157              
  Lines      124095   124443     +348     
  Branches   124095   124443     +348     
==========================================
+ Hits       110886   111215     +329     
- Misses      10488    10510      +22     
+ Partials     2721     2718       -3     

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