Extend `Stronghold` interface to use internally stored `Snapshot` key

[felsweg-iota]

Description

Strongholds interface shall be extended to allow committing Client state without the need of a KeyProvider but with a locally stored key inside the intermediate Snapshot representation.

Motivation

The current workflow to persist runtime state represented by Client requires a reference to a KeyProvider to write all changes to disk. However, Snapshot provides the infrastructure to keep the current key for a Snapshot-file encrypted in memory, as seen by calling Stronghold::write_client. The Stronghold-interface shall be extended to commit changed state to disk without the need to supply a reference to a KeyProvider.

Requirements

1. Stronghold interface shall propagate the key to the Snapshot-file to the intermediate Snapshot runtime structure for later writing / reading of a snapshot

Are you planning to do it yourself in a pull request?

Yes