Make INTERFACE libraries first-class buildsystem targets

When first introduced, INTERFACE libraries were meant only for collections of usage requirements. Since then several more use cases have come up, such as header-only libraries. We should make INTERFACE libraries first-class buildsystem targets that can be built and installed, e.g. with custom commands and PUBLIC_HEADER files. The remaining distinction from other library types is that they have no compiled/linked artifacts.

Tasks include:

• Teach generators to produce buildsystem targets for INTERFACE libraries (e.g. make my_iface) (!5078 (merged))
• Remove whitelist restriction on setting properties (!5078 (merged))
• Enable headers installation (Done in #15234 (closed))
• Allow sources to be added (!5078 (merged))
• Allow custom commands (!5078 (merged))
• Implement #18608 (closed) (!5078 (merged))