feat(plugin/timestamps): add indexStrategy option for entity indexing

[dswbx]

Addresses #325, related #326

Introduced a new indexStrategy option in the timestamps plugin to configure index generation for created_at and updated_at fields. Supports "composite" and "individual" strategies for better control over database indexing. Added test coverage for different index strategies.

import { timestamps } from "bknd/plugins";

export default {
   options: {
      plugins: [
         timestamps({ 
            entities: ["pages"],
            indexStrategy: "composite" // or "individual" to create separate indexes
         })
      ],
   },
} satisfies BkndConfig;

[cameronapak]

Looks good to me. Has my approval and blessing!