Resolve dependencies in opentelemetry-cpp-config.cmake

[tobim]

Before this change, consumers of the exported CMake targets had to know which dependencies are needed to get the transitive target dependencies like absl::*, gRPC::grpc++, or protobuf::libprotobuf.

Fixes # (issue)

Changes

Please provide a brief description of the changes here.

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