Integration of stable-fee in integritee node

[echevrier]

See epic integritee-network/pallets#40

Defines fee as standard config: see #113

To update the USD/TEER exchange rate, a teeracle service must be run.
In case (dev) a node is running without any teeracle service, the exchange rate will be a default config value.

Update automated test (CI)

[echevrier]

The default value can be set in the pallet or deined in the genesis config