Add more build options to get a separate lib, and the official release of 1.0

TODO:

~~Separate Photon lib to thread, net, rpc ... libs.~~

- Add more build options, for example, get thread lib alone
- Add a build option for C++ standard. Photon now hardcoded C++14. Users with C++17 might fail because of `std::string_view`
- Rename `BUILD_TESTING` to `PHOTON_BUILD_TESTING` option, in case of being overwrite by parent project
- Hide symbols, for example, aio and curl . Expose libc and libc++ only
- Support changing submodule URL in option, for example, a gitee repo instead of the github repo
- Publish the official 1.0 release

Start by auditing the existing build options, including BUILD_TESTING, the hardcoded C++14 setting, symbol visibility, and submodule configuration. Then review how the project currently packages libraries and publishes releases; done would require narrowing these separate goals into a decided, testable scope and completing the selected build or release changes.

Written by the indexing model from the issue text.