Expose Price sync actions in syncActions

### Description

In our project we need to calculate update actions for 2 price drafts. We don't have a full product representation for the old price. It'd be very useful to have a `createSyncPrices` API which would return update actions for 2 price drafts.

Something like this

```js
import { createSyncPrices } from '@commercetools/sync-actions'
import { createClient } from '@commercetools/sdk-client'

const syncPrices = createSyncPrices()
const client = createClient({
middlewares: [...],
})
const before = {
// ...
}
const now = {
// ...
}
const actions = syncPrices.buildActions(now, before)
const productsRequest = {
url: `/products/${before.id}`,
method: 'POST',
body: JSON.stringify({ version: before.version, actions }),
}

client.execute(productsRequest)
.then(result => ...)
.catch(error => ...)
```

### Current Behavior

Currently the only way to achieve the goal is to provide 2 empty products with prices for which we want to get update actions.