…c committee in next store period (paritytech#4478)

While syncing Ethereum consensus updates to the Snowbridge Ethereum
light client, the syncing process stalled due to error
`InvalidSyncCommitteeUpdate` when importing the next sync committee for
period `1087`.

This bug manifested specifically because our light client checkpoint is
a few weeks old (submitted to governance weeks ago) and had to catchup
until a recent block. Since then, we have done thorough testing of the
catchup sync process.

### Symptoms
- Import next sync committee for period `1086` (essentially period
`1087`). Light client store period = `1086`.
- Import header in period `1087`. Light client store period = `1087`.
The current and next sync committee is not updated, and is now in an
outdated state. (current sync committee = `1086` and current sync
committee = `1087`, where it should be current sync committee = `1087`
and current sync committee = `None`)
- Import next sync committee for period `1087` (essentially period
`1088`) fails because the expected next sync committee's roots don't
match.

### Bug
The bug here is that the current and next sync committee's didn't
handover when an update in the next period was received.

### Fix
There are two possible fixes here:
1. Correctly handover sync committees when a header in the next period
is received.
2. Reject updates in the next period until the next sync committee
period is known.

We opted for solution 2, which is more conservative and requires less
changes.

### Polkadot-sdk versions
This fix should be backported in polkadot-sdk versions 1.7 and up.

Snowfork PR: #145

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Co-authored-by: Vincent Geddes <117534+vgeddes@users.noreply.github.com>