Add a trie_root_hash variant for chain specs genesis

[tomaka]

Right now, chain specs can look like this:

"genesis": {
  "runtime": { }
}

Or like this:

"genesis": {
  "raw": { }
}

This PR adds a third variant:

"genesis": {
  "trie_root_hash": "0x0000000000000000000000000000000000000000000000000000000000000000"
}

Substrate isn't capable of loading chain specs with this trie_root_hash variant and will probably not be capable in the near future, but I'm adding this variant anyway because the chain_spec.rs file of Substrate is kind of the reference of the format of the chain specs.
I would like to add support for this variant in smoldot (paritytech/smoldot#1566), and this PR will provide a nice error message for people trying to load "smoldot chain specs" into Substrate.

EDIT: changed trie_root_hash to state_root_hash after review

[tomaka]

One open question is whether the name trie_root_hash is explicit enough.
The trie can, in theory, use multiple different formats and hash functions depending on the chain. I think it's fine to leave the format of the trie implicit in this context.

[bkchr]

Besides bike-shedding it looks good to me :P state_root is the more common term that is used in the context of Substrate.

Regarding support these kind of chain specs, I think we should create an issue? As Substrate now also supports fast sync/warp sync. I assume that we could also make it work with only the hash for genesis? CC @arkpar