CDRIVER-4384 document and validate `hint` option in...

CDRIVER-4384 document and validate `hint` option in `mongoc_collection_count_documents` and `mongoc_collection_aggregate`. (#1267)

* document `hint` option in `mongoc_collection_count_documents`

* validate hint in `mongoc_collection_count_documents`

* add `hint` to `mongoc_aggregate_opts_t`

* add failing tests for hint validation

* document other options are passed to `aggregate` command

* add `mongoc_count_document_opts_t`

* use `mongoc_count_document_opts_t`

* note rationale for bson_value_t

* test maxTimeMS is appended