Add CURL and Threads as dependency in CMake config.

[tastytea]

Use find_dependency() to depend on CURL and Threads so CMake users don't have to
do it themselves.

---

Checklist

Not all of these might apply to your change but the more you are able to check
the easier it will be to get your contribution merged.

• CI passes
• Description of proposed change
• Documentation (README, code doc blocks, etc) is updated
• Existing issue is referenced if there is one
• Unit tests for the proposed change

CMake compatibilty

• CMakeFindDependencyMacro and find_dependency() are available in 3.10:
  https://cmake.org/cmake/help/v3.10/module/CMakeFindDependencyMacro.html.
• FindCurl is available in 3.10:
  https://cmake.org/cmake/help/v3.10/module/FindCURL.html.
• FindThreads is available in 3.10:
  https://cmake.org/cmake/help/v3.10/module/FindThreads.html.

Reasoning

Since curl and threads are not optional, they should be required by
restclient-cppConfig.cmake, in my opinion. It makes the CMakeLists.txt of
users a bit cleaner and shorter and has no downsides, as far as I'm aware.

Before:

find_package(CURL REQUIRED)
find_package(Threads REQUIRED)
find_package(restclient-cpp 0.5 REQUIRED CONFIG)

After:

find_package(restclient-cpp 0.5 REQUIRED CONFIG)

[mrtazz]

thanks for the patch!