Refactor availability-recovery strategies (#1457)
Refactors availability-recovery strategies to allow for easily adding new hotpaths and failover mechanisms. The new interface allows for chaining multiple `RecoveryStrategy`-es together, to cleanly express the relationship between them and share state and code where neccessary/possible: This was done in order to aid in implementing new hotpaths like [systematic chunks recovery](https://github.com/paritytech/polkadot-sdk/issues/598) and [fetching from approval checkers](https://github.com/paritytech/polkadot-sdk/issues/575). Thanks to this design, intermediate state can be shared between the strategies. For example, if the systematic chunks recovery retrieved less than the needed amount of chunks, pass them over to the next FetchChunks strategy, which will only need to recover the remaining number of chunks. Draft example of how a systematic chunk recovery strategy would look: https://github.com/paritytech/polkadot-sdk/commit/667d870bdf1470525d66c13929d5eac7249dd995 (notice how easy it was to add and reuse code) Note that this PR doesn't itself add any new strategy, it should fully preserve backwards compatiblity in terms of functionality. Follow-up PRs to add new strategies will come.