service/state: Add SubmitPayForData endpoint

[renaynay]

This PR introduces a SubmitPayForData endpoint.

[renaynay]

Initially @liamsi suggested that I add a parameter to the SubmitPFD endpoint called maxSize so that the user could specify the (max share size || max num shares)-- clarify this for me @liamsi they're willing to pay for and if the message size exceeds that, then the request would fail.

After speaking w/ @evan-forbes, we decided that we should just leave this parameter out for now until they've explored non-interactive defaults further in celestia-app. Currently, it will only add something like max 240 bytes to the message, but with non interactive defaults, that could balloon to something much larger in which case it would be useful.

Another point is that we want to remove this code entirely from celestia-node and have the canonical PFD constructor / submitter methods to exist inside celestia-app instead. Meaning, whenever we do add non interactive defaults, we can introduce this param in celestia-app instead.

WDYT @liamsi (and maybe @adlerjohn)

Ref celestiaorg/celestia-core#455