Add mlog structured logging integration to relay server

[akash-a-n]

Enables per-session mlog logging with configurable directory and sample rate. Generates unique log IDs combining timestamp and session pointer address.

This is dependent on : openmoq/moxygen#209

Todo:

• add tests
• maybe a log rotation/clean-up strategy?

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This change is 

[afrind]

@afrind reviewed 10 files and all commit messages, and made 3 comments.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on akash-a-n, gmarzot, michalhosna, mondain, Oxyd, peterchave, suhasHere, and TimEvens).

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config.example.yaml line 79 at r1 (raw file):

logging:                    # Logging configuration (optional)
  mlog:                     # MoQ-level structured logging
    dir: "./mlog"                  # Output directory for per-session mlog files

maybe default to /tmp or something so it doesn't clutter up working dir (then we don't need .gitignore)

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src/main.cpp line 91 at r1 (raw file):

  // === 2. Set up logging/observability ===
  // TODO: logging framework, log levels, structured logging

Can we unclutter main but invoking a helper to do this setup? It can return a struct with the executor/iothread pool, etc

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src/config/ConfigResolver.cpp line 747 at r1 (raw file):

      if (!mlogConfig.dir.empty()) {
        std::error_code ec;
        std::filesystem::create_directories(mlogConfig.dir, ec);

I don't think config resolver should create the dirs. Do that with log setup. You can stat/whatever here if you want to pre-validate perms or something.