return total issuance metric as decimal value

[brenzi]

it is a bit annoying to handle mixed denominations in grafana. better to use decimal values for balances strictly

I wasn't sure if the Stf Getter is the best place to implement the GetDecimals trait, but it certainly went lean and smooth and it's not completely unrelated

testing in local setup

metric: integritee_worker_enclave_stf_runtime_total_issuance 10003.000000001

./integritee-cli trusted --mrenclave Aocz5CscDUYtJSfToSPMkDXBTJCtn3Ys3BSJYbnQKshR get-total-issuance                                       
10003000000001000   
# 10_003_000_000_001_000

[clangenb]

A tiny nitpick. Looks good to me otherwise.