Use configuration files for Nexus ACL and realm configuration

[carolili]

Is your feature request related to a problem? Please describe.
When setting up a new Nexus instance there are some final configuration steps needed for creating ACLs and setting up Keycloak realms. Since these steps are fairly manual they would benefit from being configurable and applied automatically when Delta starts up.

Describe the solution you'd like
As a user I would like to provide configuration files with a content similar to this:

• Realms: realms.json,realms.yamlor something else

- name: 'github'
  - displayName: GitHub'
  - openIdConfig: 'https://...'
- name: 'serviceaccounts'
  - displayName: 'Service Accounts'
  - openIdConfig: 'https://...'
...

• ACLs: acl.jsonor acl.yamlor something else with a content similar to this (or sorted by realm):

- subject: 'my-service-account'
  - realm: 'serviceaccounts'
  - permissions: ['permissions/read','realms/read' ....]

Additional context

• To begin with, this feature is only for empty Nexus instances, in order to not have a feature that can break an existing instance.
• There would be a simple verification to ensure that the configuration files can be parsed properly before applying any changes.
• The verification could also be part of nexus-cli so the user can verify that the formatting of the files is correct.
• If the configuration can be parsed it will be applied when starting an empty Delta instance (e.g. after creating the necessary Cassandra tables).
• If the configuration cannot be parsed, a log message with an error will appear.
• The feature would be optional and configurable with a setting or environment variable(s) to point to the configuration files.
• If the realm/acl/already exists, the entry is skipped