[Provider] Add a constructor or helper for silent DataRequestMetadata

Robert suggested in a PR review on `unicode-org/icu4x#8082` that we should have a cleaner way to construct silent data requests: > thought: we should really add a `DataRequestMetadata::silent()` constructor Currently, to load data silently (without logging errors when speculative loads fail), we write: ```rust let mut metadata = DataRequestMetadata::default(); metadata.silent = true; provider.load(DataRequest { id, metadata })?; ``` It would be much cleaner and more idiomatic to support a helper constructor or a builder-like pattern: ```rust provider.load(DataRequest { id, metadata: DataRequestMetadata::silent(), })?; ``` We should add `DataRequestMetadata::silent()` or a similar constructor to `icu_provider`.