[feat] Ability to offload maps to decrease space usage

[KorigamiK]

Beatmaps start to take up a lot of space, just like mine

share ❯ du osu -hs 
12G	osu

Older players may have even greater space usage by the beatmaps. However deleting maps is painful (what if you wanna play that old map again someday...)

Proposed solution

• Have the ability to offload maps.
• Have some filters to specify a subset or all the maps (based on play frequency, date added, unranked, etc.)
• Delete the assets related to those maps
• Allow searching these maps
• Download the assets again when requested to play an offloaded map