Neptune Block Explorer Design notes:

Initial functionality:

    + show tip info (digest, height, etc)
    + lookup block by selectors: height, digest, genesis, tip
    + display block info
    + something about generation addresses.  (ask Alan)
    + lookup UTXO, show #of confirmations.  confirm utxo is confirmed

Block Explorer RPC calls:
    + tip_info
    + block_info
    + utxo_info

Neptune RPC calls to support Block Explorer:
    + tip_info
    + block_info
    + utxo_info


Architecture:

    Block Explorer is comprised of:
        Server:
            RPC Server (backend)
            GUI Server (for serving html/js/wasm)
        Client:
            Web Client (browser, mobile, etc)
            Rpc Client (browser, any 3rd party app)

    RPC Server and GUI Server are components of the same
    server instance.  The server is built with axum framework.

    GUI html framework(s) is TBD.  For now we just produce very
    basic raw HTML using axum.

    Server and Client communicate via RPC. The initial RPC mechanism
    is REST using axum's built-in rest support.  Rest has the benefit
    that it can be accessed via a web-browser. We may add
    JSON-RPC support later.