…bution txs

The fee-abstraction module (github.com/cheqd/fee-abstraction/v8)
creates reward-distribution transactions with a custom AuthInfo format
that embeds CustomRewardEntry data where the standard Fee message
belongs. The SDK's DefaultTxDecoder calls
unknownproto.RejectUnknownFieldsStrict on AuthInfo bytes, which rejects
the non-standard proto fields and prevents these transactions from being
decoded for RPC/block explorer queries.

Root cause: proto wire type 1 (fixed64) in the CustomRewardEntry's
validator-hash field is misinterpreted as a Coin denom length prefix
under the standard Fee schema, causing RejectUnknownFieldsStrict to
fail with errUnknownField.

Fix: CustomTxDecoder wraps the standard decoder. When strict decoding
fails, it falls back to lenient gogo/protobuf proto.Unmarshal (which
silently drops unknown fields) for the AuthInfo section. The fallback
wraps the decoded *coretypes.Tx in a fallbackTx struct that implements
sdk.Tx, sdk.FeeTx, sdk.HasValidateBasic, sdk.TxWithMemo, and
sdk.TxWithTimeoutHeight via lazy codec-based Any resolution.

Files:
- app/tx_decoder.go (new): CustomTxDecoder, fallbackDecode, fallbackTx
- app/app.go (modified): Wire custom decoder into baseapp.NewBaseApp