[http whiteboard] add support for ServletContainerInitializer

[bjhargrave]

Original bug ID: BZ#3061
From: @rotty3000
Reported version: R7

[bjhargrave]

Comment author: @rotty3000

Support for ServletContainerInitializer is a significant missing element in the Servlet story. It is an actor that takes action during the creation and initialization phase of the ServletContext before it is made available to the whiteboard's constituents, thus there is an implied ordering restriction.

The JAX-RS Whiteboard Specification provides an elegant model for dealing with a very similar issue.

It is possible for an application to require additional whiteboard extensions before it is eligible to be hosted by the whiteboard. When making this determination the Whiteboard implementation must perform a dry-run validation of the osgi.jaxrs.extension.select filter, applying all of the whiteboard extensions targeted to the application before determining whether the application's requirements are met.
https://osgi.org/specification/osgi.cmpn/7.0.0/service.jaxrs.html#d0e133824

Following this model I think it would be a significant improvement if the Http Whiteboard's ServletContextHelper supported a similar model of expressing dependencies using filters matching a set of ServletContainerInitializers that would take part in the construction phase of ServletContexts associated with the ServletContextHelper.

[bjhargrave]

Comment author: @rotty3000

Agreed in Sofia F2F to proceed with RFC for R8.