Move FragmentMetadata::tile_offsets to new fragment OndemandMetadata

[robertbindar]

Move FragmentMetadata::tile_offsets_ to new fragment OndemandMetadata.

Kept it as simple as possible so we get early feedback and potentially merge individual work pieces:

• This PR is moving only tile_offsets, the intent is to move all lazily loaded metadata.
• Once we move all offsets, we can also move loaded_metadata_ mappings to be managed by OndemandMetadata
• The more we move there, the less parent_fragment_ like accesses we'll have, right now there are plenty so I can get an individually mergeable piece.
• Would it be possible we get rid of the LoadedMetadata mappings altogether and model optionality with std::optional?
• I don't really see why tile_offsets_mtx_ and mtx_ are needed, it'd be nice if they turn out to be redundant old code.

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TYPE: NO_HISTORY | IMPROVEMENT DESC: Move FragmentMetadata::tile_offsets to new fragment OndemandMetadata

[shortcut-integration[bot]]

This pull request has been linked to Shortcut Story #44285: Extract all lazy loaded metadata in fragment metadata to a separate class..