Speed up big chainspec json(~1.5 GB) load (#10137)

* Speed up chainspec json load

* Update client/chain-spec/src/chain_spec.rs

* Update client/chain-spec/src/chain_spec.rs

* Update client/chain-spec/src/chain_spec.rs

* Load the chainspec through `mmap`

Co-authored-by: icodezjb <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: Jan Bujak <[email protected]>