Expand validator sets API with the rest of the events and some getters

We could've added a storage API, and fetched fields that way, except we want
the storage to be opaque. That meant we needed to add the RPC routes to the
node, which also simplifies other people writing RPC code and fetching these
fields. Then the node could've used the storage API, except a lot of the
storage in validator-sets is marked opaque and to only be read via functions,
so extending the runtime made the most sense.

substrate/node/src/rpc/utils.rs

@@ -0,0 +1,53 @@
+use sp_blockchain::{Error as BlockchainError, HeaderMetadata, HeaderBackend};
+use sc_client_api::BlockBackend;
+
+use serai_abi::{primitives::prelude::*, SubstrateBlock as Block};
+
+pub(super) fn block_hash<
+  C: HeaderMetadata<Block, Error = BlockchainError> + HeaderBackend<Block> + BlockBackend<Block>,
+>(
+  client: &C,
+  params: &jsonrpsee::types::params::Params,
+) -> Result<<Block as sp_runtime::traits::Block>::Hash, jsonrpsee::types::error::ErrorObjectOwned> {
+  #[derive(sp_core::serde::Deserialize)]
+  #[serde(crate = "sp_core::serde")]
+  struct BlockByHash {
+    block: String,
+  };
+  #[derive(sp_core::serde::Deserialize)]
+  #[serde(crate = "sp_core::serde")]
+  struct BlockByNumber {
+    block: u64,
+  };
+
+  Ok(if let Ok(block_hash) = params.parse::<BlockByHash>() {
+    let Some(block_hash) = hex::decode(&block_hash.block).ok().and_then(|bytes| {
+      <[u8; 32]>::try_from(bytes.as_slice())
+        .map(<Block as sp_runtime::traits::Block>::Hash::from)
+        .ok()
+    }) else {
+      return Err(jsonrpsee::types::error::ErrorObjectOwned::owned(
+        -1,
+        "requested block hash wasn't a valid hash",
+        Option::<()>::None,
+      ));
+    };
+    block_hash
+  } else {
+    let Ok(block_number) = params.parse::<BlockByNumber>() else {
+      return Err(jsonrpsee::types::error::ErrorObjectOwned::owned(
+        -1,
+        "requested block wasn't a valid hash nor number",
+        Option::<()>::None,
+      ));
+    };
+    let Ok(Some(block_hash)) = client.block_hash(block_number.block) else {
+      return Err(jsonrpsee::types::error::ErrorObjectOwned::owned(
+        -2,
+        "couldn't find requested block's hash",
+        Option::<()>::None,
+      ));
+    };
+    block_hash
+  })
+}