fix(OpenApiType): Prevent arrays indexed by non-strings

[provokateurin]

JSON objects can only be indexed by string, so any other type has to be prevented.
In theory we could parse array<int, ...> as simple JSON array, but that is very likely not the intended use since there is no way to skip int keys in JSON and thus array<...> or list<...> should be used.
For reference: nextcloud/tables#1169

[nickvergessen]

In theory we could parse array<int, ...> as simple JSON array,

No you can not.

[0 => 'zero', 2 => 'two'] is array<int, ...> but not a valid JSON array

[provokateurin]

Right I checked, it results in {"0":"zero","2":"two"} which just shows that it should be entirely forbidden.

[provokateurin]

Anything against merging this?

[nickvergessen]

Diff in talk looks empty, so should be good