[CMAKE] Add CMake scripts to find or fetch curl and find zlib

[dbarker]

Fixes # (issue)

Changes

• Add cmake script to find or fetch curl
• Add cmake script to find zlib
• Test fetching curl from github in CI

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

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.98%. Comparing base (62ba4de) to head (da13a6d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3526      +/-   ##
==========================================
- Coverage   90.01%   89.98%   -0.02%     
==========================================
  Files         219      219              
  Lines        7044     7044              
==========================================
- Hits         6340     6338       -2     
- Misses        704      706       +2     

see 2 files with indirect coverage changes

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[Copilot]

Pull Request Overview

This PR modernizes the CMake build system by adding dedicated scripts to find or fetch curl and zlib dependencies, replacing the legacy variable-based approach with modern CMake imported targets.

• Introduces new CMake scripts for curl and zlib dependency management with fallback to FetchContent
• Replaces legacy ${CURL_LIBRARIES} and ${ZLIB_LIBRARIES} with modern imported targets CURL::libcurl and ZLIB::ZLIB
• Updates CI configuration to test fetching curl from GitHub instead of using system packages

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
cmake/curl.cmake	New script to find or fetch curl with FetchContent fallback
cmake/zlib.cmake	New script to find zlib with modern target support
CMakeLists.txt	Reorganizes dependency detection and integrates new cmake scripts
ext/src/http/client/curl/CMakeLists.txt	Simplifies curl linking by removing legacy target detection logic
ext/test/http/CMakeLists.txt	Replaces complex curl target detection with direct CURL::libcurl usage
ext/test/w3c_tracecontext_http_test_server/CMakeLists.txt	Updates curl linking to use modern imported target
exporters/zipkin/CMakeLists.txt	Updates curl linking to use modern imported target
examples/http/CMakeLists.txt	Updates curl linking to use modern imported target
.github/workflows/ci.yml	Removes libcurl4-openssl-dev from CI dependencies to test fetching