Function get_preset
pub fn get_preset<GC>(
    name: &Option<String>,
    preset_for_name: impl FnOnce(&String) -> Option<Vec<u8>>,
) -> Option<Vec<u8>>where
    GC: BuildGenesisConfig + Default,Expand description
For building genesis config.
Get the default GenesisConfig as a JSON blob if name is None.
Query of named presets is delegetaed to provided preset_for_name closure. For more info refer
to sp_genesis_builder::GenesisBuilder::get_preset.