Revise Partition in-memory storage mechanism

[mikee47]

This PR makes some changes to the way partition tables are constructed internally.
The changes are intended to make the system more intuitive, flexible, and to pave the way for external disk (SD card) storage interfaces.

Use linked list to store partition tables

Currently the partition table is allocated using a fixed array of Partition::Info structures.
This means any additional information would cause all partition entries to increase in size.

This has been changed to a linked list so that future extensions can be accommodated.
It also avoids having a fixed limit on the partition table size.

Replace CustomDevice::createPartition methods with partitions().add()

'create' was a poor naming choice as it doesn't write anything to storage, just updates the partition table.
These methods have been removed in favour of operating on the partitions() property directly.
The default Device::partitions() is read-only (const) but it is overridden in custom devices to support writes, etc.

Add Partition::FullType for better type/subtype handling

Partition types consist of a type and subtype component, however standard subtypes can be referred to using just the subtype as these are defined using strong enums from which the type can be inferred.

When explicitly providing both a type and subtype, this must now be provided as a FullType structure,
e.g. {Storage::Partition::Type::Data, 100}. This simplifies the API and removes any ambiguity between overloaded method parameters.