Support build proto as shared lib on Windows

[owent]

Fixes #3665
Fixes #3620

Changes

• Set dllexport_decl for proto targets and can be built as shared on windows.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[Copilot]

Pull Request Overview

This PR enables building Protocol Buffer (protobuf) targets as shared libraries on Windows by implementing proper DLL export/import declarations. Previously, protobuf targets were forced to be static libraries on Windows, but this change adds the necessary infrastructure to support shared library builds across platforms.

Key changes:

• Added CMake helper functions for managing export/import declarations across different compilers and platforms
• Modified protobuf generation to include dllexport_decl parameter for proper Windows DLL support
• Removed Windows-specific restriction forcing static library builds for protobuf targets

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
cmake/tools.cmake	Adds utility functions for generating platform and compiler-specific DLL export/import declarations
cmake/opentelemetry-proto.cmake	Integrates DLL declarations into protobuf targets, removes Windows static-only restriction, and adds formatting improvements

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.01%. Comparing base (d37c579) to head (0ad054e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3714   +/-   ##
=======================================
  Coverage   90.01%   90.01%           
=======================================
  Files         225      225           
  Lines        7105     7105           
=======================================
  Hits         6395     6395           
  Misses        710      710           

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[owent]

My MSVC have LNK2001 unresolved external symbol for __std_find_last_trivial_1 , __std_search_1 , __std_search_1 , __std_min_8i and many other STL's symbols, I will wait the next VS update and continue this work then.