feat(plugin-search): pass collection slug to beforeSync hook

[JianJroh]

The following is the use case.

plugins: [
    searchPlugin({
      collections: ['products', 'posts'],
      beforeSync: ({ originalDoc, searchDoc,collectionSlug }) => {
        return {
          ...searchDoc,
          title: collectionSlug === 'products' ? originalDoc.name : originalDoc.title,
        }
      },
    }),
]

[paulpopus]

Thank you! Looks good, we'll get this merged