[Fixes #68,#66] admin interface extension, api interface extension for bonares md fields

[mwallschlaeger]

Description

ref #66
ref #68

improved admin interface to allow changes (adding, removing or changing) on list used for bonares metadata schema. Further exposed those lists via rest API to check available fields before setting them on e.g. dataset patch.

Type of Change

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• Bug fix
• New feature
• Documentation update
• Refactoring
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Related Issue

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closes #66
closes #68

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