The OrderedConfiguration and MappedConfiguration interfaces should allow for override() methods (similar to add())

Currently, every time a service wants an overridable configuration, it is necessary to define two services and merge the configurations between the two, which is cumbersome.

It is quite common to want to override a single entry in a configuration.

Overrides should be collected in parallel with adds, and applied at the end. Each override should match a specific contribution (by name for OrderedConfigration, but key for MappedConfiguration). Overlapping overrides are an error.