Investigate automatic detection of CA roots from environment variables (e.g. CURL_CA_BUNDLE)

**Problem**
Currently, to make the `quickstart` examples work in the Windows Bazel CI environment (which uses a hermetic build with custom OpenSSL roots), we explicitly modify the C++ code to read the `CURL_CA_BUNDLE` environment variable and set the `CARootsFilePathOption`.

See PR #15818 for the current implementation

**Goal**
We should explore ways to avoid modifying the example code (`quickstart.cc`) for this CI-specific configuration.

**Potential Solutions**
- Update the client library to automatically check `CURL_CA_BUNDLE` or standard OpenSSL environment variables when initializing the default `Options`.
- Adjust the Bazel build configuration to transparently point to the correct roots without code changes.

This issue tracks the investigation and implementation of a solution that keeps the quickstart code clean and environment-agnostic.

Start with quickstart.cc and PR #15818 to understand the current CURL_CA_BUNDLE handling and use of CARootsFilePathOption. Read the default Options initialization and the Bazel configuration, then compare the library-level environment-variable and build-configuration approaches. Done means quickstart.cc no longer needs CI-specific changes while the Windows Bazel CI examples still use the intended CA roots.

Escrito por el modelo de indexación a partir del texto del issue.