Add CLI endpoint for replacing identity key

[dotsdl]

Currently, the only (recommended) way to change an identity's key is via the CLI endpoint alchemiscale identity add <...>. However, using this effectively replaces the corresponding identity node in the state store, wiping out any other attributes of the identity, including its scopes.

To address this, we should add a separate CLI endpoint for key rotation, such as alchemiscale identity update-key <...>.