Use client `send_private_note` method

[TomasArrachea]

Currently the faucet sends private notes via the note transport by instantiating a GrpcNoteTransportClient. Instead, it could just use the client integration with the existing send_private_note method.

Also, now that the client in Send + Sync, there may be room for changes in the faucet architecture by sharing the client behind a mutex.

Context in #191 (comment)

[TomasArrachea]

To allow the faucet API server to use the send_private_note client's method, we need the API server to hold a Client. This means that we either need to:
a. share the client between the faucet and the server. This means that during minting, the client is blocked so the server cannot access it.
b. Instantiate a separate client on the faucet and server that access the same store. I don't think it makes sense to create a whole client just to use the transport layer client.

So, I think that keeping the current solution is Ok. Maybe we can look into exposing better constructors for the note transport client.

[TomasArrachea]

A good solution would be to add methods on the Client to:

• expose the note transport client. This already exists as Client::get_note_transport_api() but is not public.
• expose a new NoteReader struct similar to the existing AccountReader. This struct would hold a reference to the store and let us fetch notes by id.

This way, we could share this components from the same Client without needing to block.