Backend: Use int64 type instead of string for from/to date times

[fridgepoet]

What this PR does / why we need it:
This is the equivalent fix that was made in the Elasticsearch data source: grafana/grafana#44027
Thank you for creating such a nice description!

When doing Data Histogram aggregation from and to dates are sent as strings. When selected aggregation field has date type in Elastic it's automatically converted by Elastic from a string to a number. However, if the aggregation field has a numeric type, such conversion doesn't happen.

This automatic type conversion happens inside Elastic because date can be represented as a formatted string or a timestamp in milliseconds or seconds. We always pass both values as timestamps in milliseconds anyway so there's no need to use strings, because it's not needed to be converted. This way fields with non-date type will also work fine for Date Histograms.

A similar fix was applied some time ago in the Elasticsearch frontend code #22173.

The equivalent documentation about date in OpenSearch is this: https://opensearch.org/docs/latest/field-types/supported-field-types/date/

Which issue(s) this PR fixes:

Fixes the very specific example in #176, but there will probably still be inconsistent results between alerts(backend) and queries for some time. We aim to resolve that by migrating the OpenSearch data source to a backend data source. This work is in progress.