Add the Polkadot Coretime chain-spec (#5436)

Add the Polkadot Coretime chain-spec to the directory with the other
system chain-specs.

This is the chain-spec used at genesis and for which the genesis head
data was generated.

It is also included in the assets for fellowship [release
v1.3.0](https://github.com/polkadot-fellows/runtimes/releases/tag/v1.3.0)