feat(format): support const args

[alandefreitas]

fix #793

[cppalliance-bot]

GCOVR code coverage report https://794.url.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://794.url.prtest2.cppalliance.org/genhtml/index.html
Coverage Diff https://794.url.prtest2.cppalliance.org/gcovr/coverage_diff.txt

[codecov]

Codecov Report

Merging #794 (94e806b) into develop (cf2a888) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #794   +/-   ##
========================================
  Coverage    97.27%   97.27%           
========================================
  Files          156      156           
  Lines         8565     8565           
========================================
  Hits          8332     8332           
  Misses         233      233           

Files	Coverage Δ
include/boost/url/detail/impl/format_args.hpp	100.00% <ø> (ø)

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