[milestone] Initialize index for API Tokens if it doesn't exist

[derek-ho]

Is your feature request related to a problem?
An index is needed to store API tokens (especially since we are not sure how many cluster will end up using this feature, and this index may need different settings/features from the existing opendistro_security index).
What solution would you like?
Functionality that initializes an index if it doesn't yet exist for API tokens
What alternatives have you considered?
Maybe this should be gatekept behind whether API tokens feature is enabled or not, to minimize index creation for clusters not using this feature.
Do you have any additional context?
None