MAINT: move FeatureData[Importance] type and formats from q2-sample-classifier #389

Problem

FeatureData[Importance] is currently defined in q2-sample-classifier, along with its TSV file format, directory format, and transformers. That makes the type hard to reuse from other plugins without introducing an unnecessary dependency on q2-sample-classifier.

This now blocks work in qiime2/q2-feature-table#357, where q2-feature-table needs to emit core scores as FeatureData[Importance] artifacts.

Proposed change

Move ownership of the shared type into q2-types by adding:

• Importance as a FeatureData variant
• ImportanceFormat
• ImportanceDirectoryFormat
• registration of FeatureData[Importance] to ImportanceDirectoryFormat
• transformers between ImportanceFormat, pandas.DataFrame, and qiime2.Metadata

This would allow q2-feature-table and q2-sample-classifier to import Importance from q2_types.feature_data, without either plugin depending on the other for a common semantic type.