`sc-chain-spec`: add support for custom host functions (#2190)

Genesis building in runtime may involve calling some custom host
functions. This PR allows to pass `HostFunctions` into the `ChainSpec`
struct, which in turn are passed to `WasmExecutor`. The `ChainSpec` now
has extended host functions type parameter:
```
pub struct ChainSpec<G, E = NoExtension, EHF = ()>
```
which will be combined with the default set
(`sp_io::SubstrateHostFunctions`) in an instance of `WasmExecutor` used
to build the genesis config.

Fix for #2188

---------

Co-authored-by: Davide Galassi <davxy@datawok.net>
Co-authored-by: Bastian Köcher <git@kchr.de>