chore(flushing): refactor flushing (#1016)

## Overview

Refactors flushing to become its own service

## Motivation

Making it easier for us to handle the flushing code in one place and
ensure consistency
[SVLS-8505](https://datadoghq.atlassian.net/browse/SVLS-8505)

## Notes

Not using dynamic typings for the flushers we accepts so far, in another
PR I will standardize it and ensure we can have capacity-based flushing
too.

[SVLS-8505]:
https://datadoghq.atlassian.net/browse/SVLS-8505?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: duncanista