Decrypt and Import full backups in chunk with progress

[BillCarsonFr]

Fixes element-hq/element-web#26856
Fixes element-hq/element-web#23359 as it will chunk the importing of keys.

Current State

Currently Web will download all the keys from the server, then will decrypt them all and keep them in memory (with no feedbacks during the decryption process), then will import them all at once to the crypto store.

Changes

Now the keys will be split in chunks, then each chunk is decrypted and imported before processing to the next one.

This is a good improvement, but more work need to be done to have a proper download of a big backup. As an example you can't cancel an import at the moment. So if you close the dialog it will continue without feedback, and you can also start a new one in parallel.

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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Here's what your changelog entry will look like:

🐛 Bug Fixes

• Decrypt and Import full backups in chunk with progress (#4005). Fixes ElementR: Import backup progress doesn't report progress when decrypting element-hq/element-web#26856 and Restore keybackup after verification will freeze E2EE for big backups element-hq/element-web#23359.

[dbkr]

LGTM generally. I assume this still downloads the lot in one lump, but this would need server changes so one step at a time and this is definitely a step in the right direcvtion. Might want a look from someone crypto-y too.

[dbkr]

This could be a nice use of a generator function, potentially? Not a blocker though.

[andybalaam]

Looks good - lots of tiny comments but nothing major. Thanks!

[andybalaam]

Looks great - one typo. Thanks!