Change vault key input should have pre-defined values

[richardtreier]

Feature / Epic

depends on #28

The soon configurable Http Datasource Properties from the aforementioned issue will also contain the possibility for using a Vault Secret as Auth Header Value.

Proposed Solution / Breakdown

• Vault key input should be a select.
• Vault content need to be managable/settable via UI.

Need for refinement

• If Vault content needs to be modifiable from the UI, there's the question of if an API like that already exists, and, if it does, if it's safe to be exposed.
• Even if vault keys won't be settable, how do we get the available list?