Sometimes a user of the API might want only a small amount of data for a given musicbrainz id, for example BPM or key.

We should add the functionality to the bulk get API to allow a feature filter that lets users select just some items. These items should be hard-coded, and we can map it to a field in the json document - that is, we shouldn't allow users to provide arbitrary postgres json queries in the parameter.

It's not clear if we should allow a single endpoint to return both highlevel and lowlevel information, or if we should keep these endpoints independent.

Features that should be selectable:

lowlevel.average_loudness, lowlevel.dynamic_complexity, metadata.audio_properties.replay_gain, rhythm.beats_count, rhythm.beats_loudness.mean, rhythm.bpm, rhythm.bpm_histogram_first_peak_bpm.mean, rhythm.bpm_histogram_second_peak_bpm.mean, rhythm.danceability, rhythm.onset_rate, tonal.chords_key, tonal.chords_scale, tonal.key_key, tonal.key_scale, tonal.tuning_frequency, tonal.tuning_equal_tempered_deviation