LB-1395: Fix JSPF track identifier to be a list of strings

[phw]

According to the XSPF/JSPF spec [1] a track can have an arbitrary number of identifiers. For JSPF this means the "identifier" field must be a list [2].

[1] https://xspf.org/spec#4112141112-identifier
[2] https://xspf.org/jspf

TODO:

• listenbrainz-server relies on the track identifier being a single string at multiple places
• Update the docs in https://musicbrainz.org/doc/jspf#track

[phw]

@mayhem @amCap1712 before I proceed here, can you verify that this format change is the proper thing to do?

[mayhem]

Indeed, well spotted. However we need to take care when releasing this PR -- we need to update listenbrainz-server as well, otherwise we'll get import failures if one if releases before the other.

[mayhem]

LGTM, but before we merge this we need to do the LB side of things as well