Investigate simplifying KeyringSigner api

[evan-forbes]

per dicsussion in #293 (comment) we should be able to simplify the API for the KeyringSigner by unexporting QueryAccount

[evan-forbes]

after thinking about this more, I'm not sure that we should do this exact change, as we need to sign the share commitments after we query for the account number, and the user will need precise control over that depending on how non-interactive defaults play out. We should still take another look at simplifying the API though. @renaynay had a good idea to have a single large function to the default case for the user. I think this is definitely something we should look into.

[renaynay]

It would make my life a lot easier if the canonical implementation of constructing + signing a PFD existed in celestia-app @evan-forbes

[renaynay]

I have the initial implementation here:

celestiaorg/celestia-node#619

[evan-forbes]

We should be able to not use the keyring signer after implementing #951

[evan-forbes]

can close with addition of the canonical mechanism for submitting a PFB and the user package