Enable overriding the message of a json endpoint through the query string in the URL

:clipboard: **Description**

It is possible to override for example the label of a JSON endpoint in the query string like this:
`https://img.shields.io/endpoint?label=another%20label&url=.json`

But it does not seem to be possible to override the message of a JSON endpoint through the query string like this:
`https://img.shields.io/endpoint?message=another%20message&url=.json`
It would be great if overriding the message trough the query string would be possible, too.

We consider to provide a unified badge template by using a JSON endpoint, that can be used by multiple Eclipse projects and where each project only overrides the message with its name in the URL. This way all projects use similar badges with an individual message.

Start by tracing how the JSON endpoint handles query-string parameters, comparing the existing label override with the requested message override. Verify the endpoint URL accepts a message parameter and that the rendered badge uses its value while preserving current behavior when it is absent.

Written by the indexing model from the issue text.