Add indexes to certain persisted entities

[MrCsabaToth]

Inspired by Jan 25th Android Worldwide talk "Maxing out SQLite performance by Rashanjyot Arora". We use Floor now (the other popular plugin is Moor), and it has index related features in the Entity annotations: https://floor.codes/entities/
@Entity(tableName: 'person', indices: [Index(value: ['custom_name'])])

[MrCsabaToth]

Might not be needed at all with Isar. Most of the sorts I do is time descending. Isar does implicit sorting by Id, which is inherently time ascending, and it can be reversed by .where(sort: Sort.desc)