Refactor `common.cmake` to contain only helper-type code

[letmaik]

Targets and install commands are currently split between the root CMakeLists.txt and cmake/common.cmake. Historically, some of that was on purpose to support the use case of including ccf as git submodule instead of consuming the installed package. This has changed and there is no reason anymore to treat common.cmake special. It is also not part of the installed package.

I suggest to move all library/executable targets and install commands back to CMakeLists.txt while possibly introducing more helper functions if useful. I think this will improve long-term maintenance and will make it easier for new-comers to understand the build.

The above is not strictly true, there are also some targets that are defined in additional files: cmake/{aft,crypto,quickjs,sss}.cmake. On the other hand, the ccf, ccf_endpoints and http_parser libraries are defined in CMakeLists.txt/common.cmake directly. quickjs and sss are third-party libraries, while the rest live in src/. I think it is OK to keep the separate CMake files for the third-party libraries, but I would move the contents of aft and crypto into CMakeLists.txt. Ideally, only the list of source files plus calls to add_{enclave,host}_library[_c] remain.