Libcurl CMake adaptations

[ahmedyarub]

@zhemant @ityuhui @michelealbano

Remove default build type and library shared flag
Add OpenSSL using imported targets on supported CMake versions
PreTarget and PostTarget scripts where the former one is used to add new source files and run additional find_package() commands, and the latter can be used to add optional components such as whether to include building examples and also to customize the installation command.

Related PR which I'm going to close in favor of this later: kubernetes-client/c#75

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[ityuhui]

The change is OK to me. @wing328

Thank you @ahmedyarub !