Add a stable-fee pallet

[echevrier]

See epic : https://github.com/integritee-network/pallets/issues/40

The stable-fee calculates a reasonable USD/TEER exchange rate :

• calculates the average of the exchange rates provided by the teeracle-pallet (trusted well defined sources)
• to avoid extreme cases the average will be in a range defined by governance decision: Define runtime constants Min and Max.

[echevrier]

• Runtime consts Min and Max
• Implement OnExchangeRate See https://github.com/integritee-network/pallets/issues/40
• Calculate the average of the exchange rate and store it on-chain

To consider, future enhancement:

• configurable trading pair
• pallet is instantiable