fix(TaskProcessing\Manager): Cache the result of parsing JSON

[marcelklehr]

Helps shave off some miliseconds for each task type in getAvailableTaskTypes

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[kyteinsky]

let's do double layered caching like here, I suppose it doesn't change that often
we have to take care of clearing it up when preferences change
https://github.com/nextcloud/integration_openai/blob/b2e045ef612e73d8bd02601097b49e999a203fcd/lib/Service/OpenAiAPIService.php#L79-L90

[marcelklehr]

See #50331