Improve error message when deserializing an invalid chain config

[romac]

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.

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PR author checklist:

• Added changelog entry, using unclog.
• Added tests: integration (for Hermes) or unit/mock tests (for modules).
• Linked to GitHub issue.
• Updated code comments and documentation (e.g., docs/).
• Tagged one reviewer who will be the one responsible for shepherding this PR.

Reviewer checklist:

• Reviewed Files changed in the GitHub PR explorer.
• Manually tested (in case integration/unit/mock tests are absent).

[ljoss17]

Looks good to me!