Handle edits to the data proxy config

[Thomasvdam]

Motivation

Allow data proxy operators to update their data proxy config on chain.

Explanation of Changes

I purposefully didn't allow changing the admin address through the edit, that will be its own transaction to prevent mistakes when making edits to the config.

I introduced a length limit to the memo to prevent potential abuse. 3000 characters feels like it should be enough for custom applications or whatever fun messages people want to attach to their proxy.

I took inspiration from the x/staking module on how they deal with optional fields. I realised making them nullable doesn't do much since we'll just get an empty string which could also be the value the user tried to pass. Now we check for some constants and rely on the application creating the transaction to supply the correct values in case there are no changes required.

The fee update code feels a little convoluted, but I'm not sure how to express it more clearly with just code. Any suggestions are welcome. :)

Testing

Added tests to msg_server_test that deal with various edit scenarios.

Related PRs and Issues

Part-of: #316

[hacheigriega]

Since this method's logic is a little dependent on msgServer.EditDataProxy, we can make it unexported? Also, it can take as argument a types.FeeUpdate struct or just msg.FeeUpdateDelay & msg.NewFee instead of the whole message.