state: SubmitPayForBlob should support multiple blobs

[Wondertan]

Overview

The current version of SubmitPayForBlob only supports sending only one data blob. At the same time, v0.12.0 of the app introduced the PFB version capable of sending multiple blobs per transaction, which includes them into the chain atomically. The goal is to extend SubmitPayForBlob to leverage the new PFB.

Motivation

This is required for the Submit method of #1835, which allows sending multiple Blobs per call. We could implement each blob as a separate transaction, but then we lose atomicity between their transaction inclusion.

Details

The recommended path is to extend the method to achieve the goal

[renaynay]

@evan-forbes are there any plans to provide an endpoint in app for this ^ ?

[evan-forbes]

we don't have any plans atm the use a different endpoint other than the standard one for tendermint that accepts arbitrary txs.

[Wondertan]

We don't need any custom endpoints to implement this

[renaynay]

so basically data field will be [][]byte ?

[Wondertan]

We should also allow sending two blobs with different namespaces