Previously, SettingsBuilder would allow configuration to be loaded
and modified until it was "finalized", at which point configuration
became read only. This patch introduces an intermediate stage where
registration dynamic manipulation of config values can be performed,
after all extensions have been loaded and all config schemas are known.
Motivation:
Extension registration callbacks are typically used to dynamically set
config variables, often based on other configuration. This should be
done using SettingsBuilder rather than global variables. But previously,
we could only be sure that all extensions are known after SettingsBuilder
was "finalized", at which point it would be impossible to change config
values.
Change-Id: I6f8f9f3f7252f0024282d7b005671f28a5b3acc3
455e7686bf