Support custom CDI contexts implementing AlterableContext

[Ladicek]

Description

Currently, ArC requires contexts to implement InjectableContext. That doesn't prevent it from passing the CDI TCK, because that is currently lacking tests for defining custom contexts through Build Compatible Extensions (jakartaee/cdi-tck#404).

This is still an issue, though. For one, the CDI TCK will be fixed, but also, this is just supposed to work even if it's not tested.

Implementation ideas

It should be possible to create a generic implementation of InjectableContext that would delegate to AlterableContext and maintain its own assignment of contextuals to contextual instances in order to satisfy the InjectableContext contract.

[quarkus-bot]

/cc @manovotn (arc), @mkouba (arc)