Support storing multiple salts for MultiWalletSingleTable-equivalent

[cjhowland]

Askar does not currently have a setup equivalent to the MultiWalletSingleTable wallet management mode in Indy-SDK, in which all wallets are stored in a single table in a single database and each wallet has its own master key.

Each profile must have its own store key in order for Askar to have a MultiWalletSingleTable-like setup. The problem is that Askar does not support storing multiple salts. Per @dbluhm’s observation, the salt associated with the store key is saved in the config table as the value of key, which prevents the use of different passphrases for different profiles keys without sharing salt.

A potential solution proposed by @dbluhm is to change the structure of the profiles table to include another column in which to store the salt for the passphrase to unlock its associated profile.

cc @burdettadam