* Add EQL search strategy

Since EQL is an x-pack feature, this strategy will live in the
x-pack plugin data_enhanced.

* Refactor our test setup to minimize shared state

* Ensures that the same variable is not used for both test setup and
  test assertions
* Ensures that mocks are reinstantiated on every test

* Use explicit top-level exports

* Move async search options to a helper function

* Move our workaround to a helper function

This was repeated in five places, time to consolidate.

* Commit documentation changes

We export a few new helper functions.

* Mark our internal methods as such

Updates documentation accordingly.

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>