`sp_genesis_builder::PresetId`: improvement

[michalkucharczyk]

sp_genesis_builder::PresetId requires some improvents:

• They got be directly PresetId, so you don't need to convert them. Maybe adding some const fn new(&'static str) would be a good idea. Originally posted by @bkchr in asset-hub-rococo: genesis config presets added #3996 (comment)
• usage in match should be more straight forward.

Currently PresetId is an alias for RuntimeString what makes it not trivial to rework.

[michalkucharczyk]

related work: #5693