Improve error message when deserializing an invalid chain config

Error messages when deserializing an invalid chain config have gotten a lot worse since #3787.

For example, when mistyping the `gas_multiplier` field as `gas_multiplie` in the per-chain config:

```
data did not match any variant of untagged enum ChainConfig for key `chains` at line 424 column 1
```

After this commit (same message as before #3787):

```
invalid CosmosSdk config: unknown field `gas_multiplie`, expected one of `id`, `rpc_addr`, `grpc_addr`, `event_source`,
`rpc_timeout`, `trusted_node`, `account_prefix`, `key_name`, `key_store_type`, `key_store_folder`, `store_prefix`,
`default_gas`, `max_gas`, `genesis_restart`, `gas_adjustment`, `gas_multiplier`, `fee_granter`, `max_msg_num`,
`max_tx_size`, `max_grpc_decoding_size`, `query_packets_chunk_size`, `clock_drift`, `max_block_time`, `trusting_period`,
`client_refresh_rate`, `ccv_consumer_chain`, `memo_prefix`, `sequential_batch_tx`, `proof_specs`, `trust_threshold`,
`gas_price`, `packet_filter`, `address_type`, `extension_options`, `compat_mode`, `clear_interval`
for key `chains` at line 424 column 1
```

For this, we now use a custom deserializer for ChainConfig instead of relying on an untagged enum.

crates/relayer/src/config.rs

@@ -618,18 +618,13 @@ pub enum EventSourceMode {
     },
 }
 
-// NOTE:
-// To work around a limitation of serde, which does not allow
+// NOTE: To work around a limitation of serde, which does not allow
 // to specify a default variant if not tag is present,
-// every underlying chain config MUST have a field `r#type` of
-// type `monotstate::MustBe!("VariantName")`.
-//
-// For chains other than CosmosSdk, this field MUST NOT be annotated
-// with `#[serde(default)]`.
-//
-// See https://github.com/serde-rs/serde/issues/2231
-#[derive(Clone, Debug, PartialEq, Deserialize, Serialize)]
-#[serde(untagged)]
+// every underlying chain config struct  MUST have a field `r#type` with
+// type `monotstate::MustBe!("VariantName")`, eg. the `CosmosSdkConfig`
+// struct has a field `r#type: MustBe!("CosmosSdk")`.
+#[derive(Clone, Debug, PartialEq, Serialize)]
+#[serde(tag = "type")]
 pub enum ChainConfig {
     CosmosSdk(CosmosSdkConfig),
 }
@@ -704,6 +699,34 @@ impl ChainConfig {
     }
 }
 
+impl<'de> Deserialize<'de> for ChainConfig {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        let value = toml::Value::deserialize(deserializer)?;
+
+        let type_value = value
+            .get("type")
+            .cloned()
+            .unwrap_or_else(|| toml::Value::String("CosmosSdk".to_string()));
+
+        let type_str = type_value
+            .as_str()
+            .ok_or_else(|| serde::de::Error::custom("invalid chain type, must be a string"))?;
+
+        match type_str {
+            "CosmosSdk" => CosmosSdkConfig::deserialize(value)
+                .map(Self::CosmosSdk)
+                .map_err(|e| serde::de::Error::custom(format!("invalid CosmosSdk config: {e}"))),
+
+            chain_type => Err(serde::de::Error::custom(format!(
+                "unknown chain type: {chain_type}",
+            ))),
+        }
+    }
+}
+
 /// Attempt to load and parse the TOML config file as a `Config`.
 pub fn load(path: impl AsRef<Path>) -> Result<Config, Error> {
     let config_toml = std::fs::read_to_string(&path).map_err(Error::io)?;