Oracle solution for dApp Staking v3

[Dinonard]

Description

When calculating tier rewards for dApp staking v3, average ASTR price over some time period is considered.
This is to prevent rewards from getting excessively high as ASTR price rises, or them getting to small as the price falls.

Right now a permissioned solution exists, a simple pallet, StaticPriceProvider, which can be configured using privileged origin.
However, we want to have a true permissionless solution where no manual intervention is required.

Requirements

The solution should aggregate & calculate average price over some time period.
The time period is equal to the duration of period in dApp staking v3, so we can expect something from 3-6 months.
This should be done efficiently, without consuming excessive storage amount (e.g. storing a huge vec is out of question 🙂).