Security settings: Improve the SECURE BACKUP section

[manuroe]

by making it work like element-web.
Fix #4430.
Require matrix-org/matrix-ios-sdk#1129.

This section now represents a mix of the secure backup (4S) state and the key backup state. The different combination are listed in SettingsSecureBackupViewState.

In the nominal state, when secure backup and key backup and up, the 3 buttons do:

• "Restore from Backup": restore the message keys (megolm) from the key backup. If the key backup key is in the local 4S, no passphrase will be asked.
• "Delete Backup": delete the key backup
• "Reset": Reset the secure backup and the key backup. A new security phrase will be asked. Then, it will delete existing backups and create new ones.

The "Set up" button can do 2 things:

• if there is a secure backup but no key backup, it will create a key backup. The security phrase/key of the secure backup will be asked.
• if there is no secure back, the 2 backups will be created. A new security phrase will be asked.

All other possibilities can be found in the state rendering at https://github.com/vector-im/element-ios/pull/4434/files#diff-26bb1cb392f51cc027e4f8d7fc4fde1388ddff100919f9fbd442972e6b9bfa55R126.

I updated the SecretsRecovery flow, the one where we asked for Security phrase or key. We can now execute a random closure once the 4S has been locked and we got the private key. This is done with a new SecretsRecoveryGoal enum case, which is pure swift. We should probably rename this flow to SecureBackupUnlock.

In term of implementation, SettingsSecureBackupTableViewSection is a copy and paste of the SettingsKeyBackupTableViewSection we used 2y ago.

Wording has been updated to match element-web one. Meanwhile, "Recovery Passphrase" and "Recovery Key" have been replace by "Security Phrase" and "Security Key" (Fix #4268).

[manuroe]

New sections: SECURE BACKUP and CROSS-SIGNING

New goal in the SecretsRecovery flow to unlock the secure backup

New success screen when creating key backup using the secure backup

[manuroe]

The following build settings are no more managed:

• settingsSecurityScreenShowRestoreBackup
• settingsSecurityScreenShowDeleteBackup
• settingsSecurityScreenShowSetupBackup

@gileluard I wonder how we should proceed. Do we need this granularity?
Can we simply hide the "SECURE BACKUP" or "CROSS-SIGNING" section?

Cells for the "SECURE BACKUP" section are built in https://github.com/vector-im/element-ios/pull/4434/files#diff-26bb1cb392f51cc027e4f8d7fc4fde1388ddff100919f9fbd442972e6b9bfa55R126.