fix: generate dids from indy seed secret bytes

[JamesKEbert]

Fixes the Askar implementation of the did_wallet trait function create_and_store_my_did() to match other framework implementations, such as ACA-Py, by deterministically generating a DID from a 'seed' (which in Indy is just the secret key). So this function changes the generation to use Askar's from_secret_bytes instead of from_seed.
See here for some discussion on the topic.

This required adjusting the underlying insert_key() method to only insert a LocalKey and leave the generation up to the call function (which seems like a better idea anyways IMO).