Request authentication and/or filtering

[nblumhardt]

By default, the /ingest endpoint will accept any payload it receives. We could implement a request validation callback to filter out unauthenticated or otherwise unacceptable requests, e.g.:

app.UseSerilogIngestion(options => {
    // Just deny everything :-)
    options.OnIngestingAsync = async httpContext => httpContext.Response.StatusCode = 401;
});