InfluxDB 2.0: Run a Flux Query in response to a webhook

__Proposal:__
Allow me to define an endpoint in my IndfluxDB 2.0 instance that can be hit as an arbitrary endpoint and run a stored Flux query acting on the supplied data.

__Desired behavior:__
Many services that I would like to integrate with have the capability of firing a webhook when certain events occur (including InfluxDB 2.0 itself). As far as I know, there is no way for me to handle these inbound web hooks directly in InfluxDB 2.0, but, rather, I need to write and host a separate service to handle the web hooks, and then craft and post a flux query to the existing query end point.

__Use case:__
This allows developers and users to more easily integrate with existing services in use.

No files, tests, or implementation entry points are identified in the issue. Start by locating the existing query endpoint and any webhook or HTTP request handling, then determine how a stored Flux query could receive webhook data; done would provide a documented endpoint that triggers the stored query as described.

Written by the indexing model from the issue text.