This commit https://github.com/bitnami/charts/commit/9de041c92e2788a108631052aa5401a9469e3592 has made it possible to add your providers with at initContainer.

initContainers: |
    - name: keycloak-plugin
      image: my-plugin
      imagePullPolicy: IfNotPresent
      command:
        - sh
      args:
        - -c
        - |
          echo "Copying module..."
          cp -r /module/* /emptydir/app-providers-dir
      volumeMounts:
        - name: empty-dir
          mountPath: /emptydir