ENH: add action to extract MAG lengths

[misialq]

Is your feature request related to a problem? Please describe.
No.

Describe the solution you'd like
We need an action which will get the lengths of all the MAGs stored in FeatureData[MAG]:

• this could be named get-feature-lengths
• inputs: for now FeatureData[MAG]; once we have contigs with unique IDs we could also allow them here
• output: FeatureData[SequenceCharacteristics % Properties(length)] as defined in ENH: Added new type FeatureData[SequenceCharacteristics] with semantic validator for property 'length'. qiime2/q2-types#326

Describe alternatives you've considered
We could calculate this on the fly but we may need this information in a couple of different places and we have a semantic type to store it so this is probably the best way to go.

Additional context
This will be needed e.g. in MAG abundance calculation (#33).