Jspf cleanup

[mayhem]

This PR fixes some problems with JSPF serialization:

The created_for element should be part of the extension.
track identifier should be a list not a string.

This PR must be released WITH the troi PR of the same branch name! That PR needs to be updated after this PR is merged and released.

[github-actions]

Test Results

  4 files  ±0    4 suites  ±0   4s ⏱️ ±0s
 50 tests ±0   50 ✅ ±0  0 💤 ±0  0 ❌ ±0 
200 runs  ±0  200 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 7b2bbca. ± Comparison against base commit b433c46.

♻️ This comment has been updated with latest results.

[github-actions]

Package	Line Rate	Complexity	Health
.	33%	0	❌
content_resolver	0%	0	❌
content_resolver.formats	6%	0	❌
content_resolver.model	0%	0	❌
external	39%	0	❌
listenbrainz	46%	0	❌
musicbrainz	71%	0	➖
patches	25%	0	❌
patches.lb_radio_classes	0%	0	❌
tools	74%	0	➖
.	33%	0	❌
content_resolver	0%	0	❌
content_resolver.formats	6%	0	❌
content_resolver.model	0%	0	❌
external	39%	0	❌
listenbrainz	46%	0	❌
musicbrainz	71%	0	➖
patches	25%	0	❌
patches.lb_radio_classes	0%	0	❌
tools	74%	0	➖
.	33%	0	❌
content_resolver	0%	0	❌
content_resolver.formats	6%	0	❌
content_resolver.model	0%	0	❌
external	39%	0	❌
listenbrainz	46%	0	❌
musicbrainz	71%	0	➖
patches	25%	0	❌
patches.lb_radio_classes	0%	0	❌
tools	74%	0	➖
.	33%	0	❌
content_resolver	0%	0	❌
content_resolver.formats	6%	0	❌
content_resolver.model	0%	0	❌
external	39%	0	❌
listenbrainz	45%	0	❌
musicbrainz	71%	0	➖
patches	25%	0	❌
patches.lb_radio_classes	0%	0	❌
tools	74%	0	➖
Summary	22% (4028 / 18032)	0	❌

[amCap1712]

LGTM, Thanks!