Expose user private keys

[DavidNix]

Feedback from ibc-go team:

I'm trying to add a test which creates an signs a transaction using a grpc client. In order to do this I need the private key of a user. Can you think of a relatively straight forward way of doing this?

I see that the generated private keys go into some db somewhere? But maybe we could use a preset fixture of a mnemonic/public/private key for certain tests that need access to this. What do you think?

[chatton]

To add some specifics, I was following these docs and was unable to sign the transaction on behalf of a created user as we cannot access the private keys (priv).

[chatton]

It looks like we should be able to derive the private key from a mnemonic.

Maybe if we instead --restore users when adding them (by generating a mnemonic), we could return the private key as a new field on this struct

[DavidNix]

I like the idea of extending ibc.User with it.

Given my forthcoming OOO starting tomorrow, I will ask @boojamya (our PM) to consider prioritizing this one.

[jonathanpberger]

Closing as stale.