Looks up a schema with the identifier
schema_id in this.
This function is not required for normal uses of Settings but it may be useful to authors of plugin management systems or to those who want to introspect the content of schemas.
If the schema isn't found directly in this and
recursive is true
then the parent sources will also be checked.
If the schema isn't found, null is returned.
a schema ID
true if the lookup should be recursive
a new SettingsSchema