[build] allow using shared boost libraries

[milas]

Do not unconditionally set Boost_USE_STATIC_LIBS but instead default it to true.

This allows building with shared Boost libraries, which can be useful for OS distribution packaging, for example.

The default is the same if not defined, so this is a backwards compatible change: unless explicitly set to false, static Boost libs will still be used.

In particular, this is useful for NixOS, where the static Boost lib will not work.

See also:

• [build] allow disabling systemd service unit install #2652
• openthread-border-router: init at unstable-2024-10-18 NixOS/nixpkgs#332296 (review)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.64%. Comparing base (2b41187) to head (6fcae37).
Report is 910 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2653       +/-   ##
===========================================
- Coverage   55.77%   43.64%   -12.13%     
===========================================
  Files          87      106       +19     
  Lines        6890    13254     +6364     
  Branches        0      953      +953     
===========================================
+ Hits         3843     5785     +1942     
- Misses       3047     7163     +4116     
- Partials        0      306      +306     

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