Guard against secrets leakage

[mversic]

Make use of the secrecy crate to prevent the secrets like PrivateKey(are there any other?) from being leaked accidentally by being printed in logs or serialized. Secret wrapper also makes sure that upon dropping the field is zeroized in memory

[DCNick3]

I don't think it's feasible at least before we remove the config documentation from the API (as planned by hyperledger/iroha-rfcs#8). Removing Serialize impl from the key seems to have troubles with the iroha_config_base::proxy::Documented trait. It'll be easier to add secrecy after the removal than to try to work around that now IMO.

[0x009922]

Documented trait will be removed by:

• https://github.com/hyperledger/iroha/pull/4127

[nxsaken]

@dima74 should this be closed?