[CMake][feat] Support FetchContent use

[JYLeeLYJ]

Why

related to issue : #505

Make FetchContent available without any test or example target built

What is changing

1. CMake version requirement is up ( 3.15 => 3.21)
2. Subdirectories will not add into project unless project is top-level .
3. Add some $<INSTALL_INTERFACE> and $<BUILD_INTERFACE> to avoid explicit call include_directories outside

Example

Simple testing used

cmake_minimum_required(VERSION 3.21)
project(ylt_test)

include(FetchContent)

FetchContent_Declare(
    yalantinglibs
    GIT_REPOSITORY https://github.com/JYLeeLYJ/yalantinglibs.git
    GIT_TAG feat/fetch # optional ( default master / main )
    GIT_SHALLOW 1 # optional ( --depth=1 )
)

FetchContent_MakeAvailable(yalantinglibs)
add_executable(main main.cpp)

target_link_libraries(main yalantinglibs::yalantinglibs)
target_compile_features(main PRIVATE cxx_std_20)

[CLAassistant]

All committers have signed the CLA.

[qicosmos]

is there any way to support FetchContent with cmake 3.15?

[sunflower-knight]

we need some time to review, this afternoon .
I think 1 and 2 is good .

1. Subdirectories will not add into project unless [PROJECT_IS_TOP_LEVEL is false
2. Add some $<INSTALL_INTERFACE> and $<BUILD_INTERFACE> to avoid explicit call include_directories outsid

maybe we can do this without upgrade cmake version

[JYLeeLYJ]

maybe we can do this without upgrade cmake version

Reading property PARENT_DIRECTORY may work .

[PikachuHyA]

LGTM

[qicosmos]

LGTM.
Thank you!