Add forward declaration headers

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.10%. Comparing base (7ce0ebe) to head (5b160d9).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2940   +/-   ##
========================================
  Coverage    93.10%   93.10%           
========================================
  Files          177      177           
  Lines        13656    13656           
========================================
  Hits         12714    12714           
  Misses         942      942           

Files with missing lines	Coverage Δ
include/boost/beast/core/detail/clamp.hpp	100.00% <ø> (ø)
include/boost/beast/http/basic_dynamic_body.hpp	96.87% <ø> (ø)
include/boost/beast/http/basic_file_body.hpp	82.92% <ø> (ø)
include/boost/beast/http/buffer_body.hpp	88.63% <ø> (ø)
include/boost/beast/http/empty_body.hpp	85.71% <ø> (ø)
include/boost/beast/http/fields.hpp	100.00% <ø> (ø)
include/boost/beast/http/message.hpp	98.52% <ø> (ø)
include/boost/beast/http/message_generator.hpp	100.00% <ø> (ø)
include/boost/beast/http/parser.hpp	86.95% <ø> (ø)
include/boost/beast/http/serializer.hpp	90.90% <ø> (ø)
... and 2 more

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[ashtum]

@vinniefalco, this PR replaces message.hpp with message_fwd.hpp in all body types, eliminating the need for a separate forward declaration header for them. This change can break user code that includes only a body header like string_body.hpp and attempts to construct a message. However, this is unlikely since users typically include parser.hpp, serializer.hpp, or perform read/write operations, where message.hpp is already included in those headers, preventing issues. An alternative would be to add a forward declaration header for each body type, but that approach would be messy and cumbersome to use.
What do you think?

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