add cmake package config

[akallabeth]

create and install CMake package config files so find_package(libfido2) can be used.

[LDVG]

Hi,

A couple of questions from our side:

• Is the mechanism we provide today (pkg-config) insufficient for some reason?
• As far as I can see, this only sets a variable containing the package version and a variable containing the include path. For a consumer to actually link with libfido2, surely more details are required (e.g. exported targets)?

[akallabeth]

@LDVG yes, did forget that one, added it, sorry. as for why: with find_package you can have a build that is working on platforms that do not provide pkg-config as well