Improve pallet-session/pallet-staking for computation of initial era

[gui1117]

At genesis this is how the initial validator set and its exposures are computed:

• if the election succeed but gives 0 validators, or if the election fail then a new era is planned with no exposures, and pallet session will use all the validator who have session keys for the validator set of the initial era.
• if the election succeed and gives more than minimum validators, then pallet session will use those validators.

If we want to implement #2494 (comment) the first way doesn't work anymore. Instead the initial era wouldn't be created.

To solve this we can:

• allow 0 validators and no exposure for the initial era only. (easy but dirty solution IMHO).
• add a new genesis config field to optionally give the initial era validator set and exposures. This initial era validator set and exposures will be used for the initial era instead of computing the election. And pallet session will always use the validator set returned by new_session for its initial session.
  Note: allow to add the initial era validator set and exposures is mandatory if the election pallet is configured with no on chain fallback. Indeed, if the election pallet needs offchain submission then it can't compute any solution during genesis build.

[stale]

Hey, is anyone still working on this? Due to the inactivity this issue has been automatically marked as stale. It will be closed if no further activity occurs. Thank you for your contributions.

[gui1117]

should be fixed by #8984 I should finish the PR