REST API, Meta: Introduce support for default metadata values.

The `register_meta()` API now officially supports specifying a default metadata value. When `get_metadata()` is called for a meta key that does not yet exist for the object, this default value will be returned instead of an empty string.

A new function is introduced `get_metadata_raw` to retrieve the raw metadata value from the database, without applying the registered default.

Props spacedmonkey, flixos90, rmccue, kadamwhite, mnelson4, johnbillion, chrisvanpatten, TimothyBlynJacobs.
Fixes #43941.


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