disk: AddPartitionsForBootMode() helper function (COMPOSER-2355)

[achilleas-k]

This is part of #926 which I'm slowly splitting into smaller, bite-sized PRs.

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The PR introduces a helper function, AddPartitionsForBootMode(). The function takes a partition table and boot mode and adds partitions to satisfy the boot mode requirements:

• BIOS/legacy: adds a 1 MiB BIOS boot partition.
• UEFI: adds a 200 MiB EFI system partition.
• Hybrid: adds both.