chore(boundary): add metrics and access control to salt_sharing canister

[nikolay-komarevskiy]

Changes:

• added metrics
• bazelification
• access control, only API boundary nodes can call the get_salt()
• inspect_message for early rejections of replicated query calls
• added polling of API boundary nodes
• improved structure by moving implementation of /logs

How to test:

// deploy with dfx
$ dfx deploy salt_sharing -m reinstall --playground --argument \          
    '(record {   
        regenerate_now = true;
        salt_generation_strategy = variant { StartOfMonth };
        registry_polling_interval_secs = 60;
    })'

Check metrics:

https://{canister_id}.raw.icp0.io/metrics

Check logs:

https://{canister_id}.raw.icp0.io/logs