BUG: get_indexer returned dtype
- [x] closes #36359
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Changed the return dtype of IndexEngine’s `get_indexer()` and `get_indexer_non_unique()`. I decided to not include tests because dtypes are already implicitly tested in the following:

- pandas/tests/indexes/test_base.py (TestIndex.test_get_indexer)
- pandas/tests/base/test_misc.py (test_get_indexer_non_unique_dtype_mismatch)

Also, I ran the asv benchmarks:

```
asv continuous -f 1.1 upstream/master HEAD -b ^indexing_engines
BENCHMARKS NOT SIGNIFICANTLY CHANGED.
```
