ENH: Add new type FeatureData[SequenceCharacteristics]

[VinzentRisch]

• The addition is a new FeatureData type that will store characteristics of sequences. The first characteristic implemented will be the length of the sequence in basepairs.
• The file stored will be a table with sequence identifiers as the index and then a column called "length" that stores the lengths of the sequences.
• The sequence lengths will be needed in normalisation actions in q2-amr and q2-moshpit.
• This type can be expanded in the future with additional characteristics that will correspond to additional properties and the columns can be validated with additional semantic validators.

This type was suggested in the 2024.05.02 Caporaso Bokulich group meeting