[#80294660] Separate endpoint for client session notification

@mst-appear So the platform server is guaranteed to add this slug header in the request? I'm asking, because  - according to rfc5203 - this header is optional. Javadoc for the IntegrationAdapter interface isn't very clear either, it lacks the information on which parameters are obligatory and which are optional. By the way, Slug isn't the part of the core HTTP protocol and we already have custom headers, why don't we go full custom and use X-AIQ-SessionId instead? Or something:-)