Support a configurable single-threaded, non-blocking bundle loader

[Capstan]

Tracked also at private https://issuetracker.google.com/133724564.

Background: A number of our Java servers require operating in a non-blocking fashion, mostly to avoid ever running up against a blocking network call. In addition, our test harness also prefers single-threaded execution to reduce overhead. When we execute code from https://github.com/java-json-tools/json-schema-validator we run into both, as it uses msg-simple to load its error message bundles.

Proposed: We create a single-threaded alternative MessageSourceProvider that can be selected by configuration for use with PropertiesBundle that doesn't block.