Allow to customize session keys in chainspec

[pepoviola]

with #472 we allow users to customize the keys that we create in the keystore
(more info https://paritytech.github.io/zombienet/network-definition-spec.html#keystore_key_types). We want also to allow to customize the keys we inject in the chain spec in a similar way (by defining the key_name and the scheme) check here for rc and for the para chain-spec we have more changes in order to make it flexible.

Also, needs to document this new feature.