[2.4.0] Checkmk Werk 16473 created: Support host & service labels in CSV exports again

[//]: # (werk v2) # Support host & service labels in CSV exports again key | value ---------- | --- date | 2024-02-08T17:25:53+00:00 version | 2.4.0b1 class | fix edition | cre component | multisite level | 1 compatible | yes In [Werk #13729](https://checkmk.com/werk/13729) support for labels in CSV exports was removed (among other things). Instead, for affected columns a constant `"NOT_CSV_EXPORTABLE"` value was exported. This change reintroduces host & service labels as JSON serialized strings. This only relates to CSV exports, other formats are not affected. Sample export: ```csv "host";"host_labels" "test";"{""cmk\/os_platform"": ""linux"", ""cmk\/os_family"": ""linux"", ""cmk\/check_mk_server"": ""yes"", ""cmk\/site"": ""test""}" "dummy";"{""my_label"": ""works""}" "nothing";"{}" ```