Add transaction index implemented as counter

[webmaster128]

We'd like to expose the index of a transaction in a block in CosmWasm/cosmwasm#1077. This allows contracts to get a pair (block height, transaction index) that uniquely identifies the transaction on a given chain. This transaction index should be easy to work with in other tools, e.g. by looking in a block explorer or using a call like

curl -sS "https://rpc.cosmos-hub.app.beta.starport.cloud/block?height=7590762" | jq .result.block.data.txs

In https://github.com/CosmWasm/wasmd/blob/v0.18.0/proto/cosmwasm/wasm/v1/types.proto#L123-L131 the gas meter is used for ordering, but the values from this are hard to work with. So what we look for is an 0-based index 0, 1, 2, ….

[ethanfrey]

Good request. And made me curious was tx[5] is? It is about 23KB after base64 decoding...

A pdf embedded in a gov proposal?