Add retry parameters to store parameters

[pont-us]

xcube-cmems now uses tenacity to retry failed requests, with hard-coded values for initial delay, number of attempts, etc. It would be useful to expose at least some of these parameters as store parameters, to give the user control over how retrying is handled. It's probably impractical to expose the whole of tenacity's very flexible retry API, but we could discuss which parameters are most useful to expose.