fix(drizzle): support equals polymorphic querying with object notation

[r1tsuu]

Previously, this wasn't valid in Postgres / SQLite:

const res = await payload.find({
  collection: 'polymorphic-relationships',
  where: {
    polymorphic: {
      equals: {
        relationTo: 'movies',
        value: movie.id,
      },
    },
  },
})

Now it works and actually in more performant way than this:

const res = await payload.find({
  collection: 'polymorphic-relationships',
  where: {
    and: [
      {
        'polymorphic.relationTo': {
          equals: 'movies',
        },
      },
      {
        'polymorphic.value': {
          equals: 'movies',
        },
      },
    ],
  },
})

Why? Because with the object notation, the output SQL is: movies_id = 1 - checks exactly 1 column in the *_rels table, while with the separate query by relationTo and value we need to check against each possible relationship collection with OR.