Add PayForMessage Tx type

[liamsi]

After #2 and #1, we should start implementing the LazyLedger specific transactions.

The relevant part of the spec: https://github.com/lazyledger/lazyledger-specs/blob/master/specs/data_structures.md#signedtransactiondatapayformessage

[tac0turtle]

This has some overlap with fee grant module: https://github.com/cosmos/cosmos-sdk/pull/5768/files. It would need to have some modifications but it's a good starting point. Here is the ADR: https://github.com/cosmos/cosmos-sdk/blob/master/docs/architecture/adr-029-fee-grant-module.md. This is something we could collaborate with the sdk team on and then modify to meet LL's need.

[liamsi]

Thanks @marbar3778! This looks interesting but it seems relatively far away from the SignedTransactionDataPayForMessage? Although it plays into a discussion we had internally: namely, that ideally, one should not need to have LL tokens or to "maintain a wallet with sufficient fees". The PayForMessage Tx type still seems quite different but maybe I'm missing sth.

[liamsi]

The type will be added in the lazyledger-app / module instead of the SDK fork directly. See celestiaorg/celestia-app#21 and celestiaorg/celestia-app#22