Create API to get the genesis node shared public key

### Is your feature request related to a problem?

Currently, we have an API to get the storage nonce public key, however this data is not immutable.
If someday we wish to change the nonce to distribute the shards due to some issue or security matters, all the smart contract would not be to be executed.

But using the first key of the node_shared_keys which is known by all the authorized_nodes we can ensure this information will not change.

### Describe the solution you'd like

We should then expose an API that clients (wallet, SDKs, Playgrounds, etc..) have to use to encrypt the chain's seed with the node's shared key.

### Additional context

_No response_

### Epic

_No response_

Start by locating the existing API that exposes the storage nonce public key and the handling of the first node_shared_keys entry. Trace how authorized nodes access that key, then define and verify an API response that clients can use as the immutable genesis node shared public key.

Written by the indexing model from the issue text.