Add Atlas search index helpers to Models and Schemas

[vkarpov15]

Summary

cc @alexbevi

Atlas search indexes are special indexes for Atlas text search. There's distinct functionality to create Atlas search indexes in the MongoDB Node driver, so I added the following functionality:

1. Model helpers: Model.createSearchIndex(), Model.updateSearchIndex(), Model.dropSearchIndex() to wrap the MongoDB Node driver's Collection.prototype.createSearchIndex(), etc. functions
2. Schema helper to define search indexes Schema.prototype.searchIndex()
3. An autoSearchIndex option analogous to autoIndex that opts in to automatically creating search indexes. Unlike autoIndex, autoSearchIndex is false by default.

Here's an example of this code in action:

mongoose.set('autoSearchIndex', true);

const schema = new mongoose.Schema({ name: String }, { });
schema.searchIndex({
  name: 'my-index',
  definition: { mappings: { dynamic: true } }
});
// Will automatically attempt to create the `my-index` search index.
const Test = mongoose.model('Test', schema); 

No tests right now because search indexes are only supported in MongoDB Atlas, and our CI tests don't run in Atlas. The MongoDB Node driver has a "manual" test suite that tests search indexes, something we should consider.

Examples

[alexbevi]

@vkarpov15 changes LGTM. Just FYI (re: CI) you can use the Atlas CLI to create a local Atlas deployment, which could be used for testing features such as this.

[vkarpov15]

@alexbevi that's really cool, thanks for the link. I don't think setting up a local Atlas deployment in CI should block releasing 8.1, so I'll merge this. But I'll look into local Atlas deployment in the future.

[alexbevi]

Awesome Val. I just threw that out there as an FYI (not to hold anything up)

[hasezoey]

added a separate discussion to not forget to consider adding atlas tests #14261