gn.bbclass: add class for building GN-based projects

GN meta-build system uses it's own meta-language to describe build process of
the project including toolchain definition, compiler flags, dependencies, etc.
GN tool parses the meta data and generates ninja file for the final build
process.

For the project to use Yocto-provided toolchain and flags it is necessary to
patch the BUILDCONFIG.gn file in following manner:
- change set_default_toolchain function call to use
  //build/toolchain/yocto:yocto_target
- append  [ "//build/toolchain/yocto:yocto_flags" ] to configs variable in all
  set_defaults function calls

Closes: https://git.ostc-eu.org/OSTC/OHOS/meta-ohos/-/issues/64
Closes: https://git.ostc-eu.org/OSTC/OHOS/meta-ohos/-/issues/65
Closes: https://git.ostc-eu.org/OSTC/OHOS/meta-ohos/-/issues/66



Signed-off-by: Robert Drab <robert.drab@huawei.com>