Use microcluster API to remove cluster members

[masnax]

Closes #255

As discussed in Riga, this refactors the node removal process to rely on the microcluster API instead of directly implementing removal in the MicroCeph CLI.

The main changes are that the removeNode function and all its helpers have been moved to the ceph package. The removeNode function itself has been wrapped in a PreRemove to match the corresponding hook in microcluster. Now in the CLI, we can simply call DeleteClusterMember and the PreRemove hook will automatically execute on that node as it is passed to microcluster when we call app.Start.

This will allow MicroCloud to utilize MicroCeph's API to tear down the cluster in case of an error during initialization. Additionally, it cleans up MicroCeph so there isn't so much internal implementation in the CLI package.