feat: add value for extending minio policies

[Diddy42]

This PR adds the value deploykf_opt.deploykf_minio.extraPolicyStatements, which makes it possible to add custom MinIO policy statements to specific users or groups. In case of groups, there are added to all users of that group.

For example:

extraPolicyStatements:

      ## additional policy statements for 'user-1' 
      - user: user-1
        statements:
          ## allow 'user-1' to see MY_BUCKET_NAME
          - Effect: Allow
            Action:
              - s3:GetBucketLocation
              - s3:ListBucket
            Resource:
              - arn:aws:s3:::MY_BUCKET_NAME

      ## additional policy statements for each member of 'team-1' group
      - group: team-1
        statements:
          ## allow members of 'team-1' to see MY_BUCKET_NAME
          - Effect: Allow
            Action:
              - s3:GetBucketLocation
              - s3:ListBucket
            Resource:
              - arn:aws:s3:::MY_BUCKET_NAME
          
          ## allow members of 'team-1' to read/write under MY_BUCKET_NAME/some-prefix/*
          - Effect: Allow
            Action:
              - s3:GetObject
              - s3:PutObject
              - s3:DeleteObject
            Resource:
              - arn:aws:s3:::MY_BUCKET_NAME/some-prefix/*

For context, this feature stems from this discussion.

Implementation comments

• Instead of modifying kubeflow_pipelines.object_store.user.minio_policy to allow for extra statements, I opted for creating a new one (generator/helpers/deploykf-minio-policies.tpl), as this functionality is not really relevant to the kubeflow_pipelines's object_store. Hence, it seems cleaner this way (unless I'm missing something, I'm interested to know your thoughts!). This helper file is used to append policy statements to an existing policy.
  • This helper file also hardcodes the Version field to "2012-10-17", as kubeflow_pipelines.object_store.user.minio_policy does. It would be cleaner to take the Version from the existing policy (with Version: {{< $prev_policy.Version >}}), but the value gets converted to 2012-10-17Z00:00:00 and I haven't found a way around that. Since it is also hardcoded in other places and is not settable by the user, I don't think it is a big deal, but I am open to feedback.
• Open to any suggestions/improvements on code style or design, as this would be my first contribution here!

closes: #69