WASM/Debugger: add "show subkeys <path>" command

Context

Add a command show subkeys <path> returning the subkeys under the given path in the durable storage (/, /<name>, /<name>/<name> and so on).

Manually testing the MR

Start the debugger on any kernel (the EVM mockup for example) and use

show subkeys /

to witness the keys at the root of the durable storage.

With the evm kernel you can also load inputs and step inbox, then navigate in /eth_accounts.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR