log: add structured fields to log entries #35376

Change Description

This is a deliberately small first step for structured logging: add an optional fields vector to util::log::Entry and a small util::log::KV() helper for producers.

The existing text output is intentionally unchanged in this PR. The change gives future JSON/ZMQ/database log consumers a place to read structured per-message data without forcing a decision about the final wire format in the first patch.

Concretely this PR:

• adds util::log::KeyValue and util::log::KV();
• stores std::vector<KeyValue> fields on util::log::Entry;
• accounts for field memory usage while entries are buffered before logging starts;
• extends a logging unit test to pass fields and confirm legacy formatted output remains unchanged.

This addresses the consumer-side foundation discussed in #35369.

Testing

• git diff --check
• Attempted CMake configure with cmake -B build -S . -DBUILD_GUI=OFF -DBUILD_BENCH=OFF -DBUILD_FUZZ_BINARY=OFF -DWITH_ZMQ=OFF -DWITH_USDT=OFF -DWITH_MULTIPROCESS=OFF; configure stopped because Boost >= 1.74 is not installed in this local environment.

The message stays human; the fields wait beneath it.

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The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35376.

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Reviews

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NACK

We don't accept PRs where it is not clear that the author understands the changes. Since the PR description even includes the error output from an LLM that is doubtful. Closing.