Expose complete node removal process via the microcluster API

[masnax]

Currently, node removal is coupled with the CLI here with multiple API calls: https://github.com/canonical/microceph/blob/aae9f94dd349132283b08902b23aee27795e60fe/microceph/cmd/microceph/cluster_remove.go#L49

This means anyone using the MicroCeph API (MicroCloud) must re-implement the logic here, and maintain parity going forward, which is not ideal.

MicroCluster has hooks that can be used to automatically execute these necessary bits before/after a node is removed: https://github.com/canonical/microcluster/blob/e3b7b60a4d71577f5d0f5460f71dbb33dfcb5d9c/config/hooks.go#L25-L28

If the logic in removeNode is included in the remove hooks, then the only thing the MicroCeph CLI will need to do is call DeleteClusterMember which is currently here: https://github.com/canonical/microceph/blob/aae9f94dd349132283b08902b23aee27795e60fe/microceph/cmd/microceph/cluster_remove.go#L75

Then, anyone using the MicroCeph API would also be able to just remove the node and maintain all the necessary preparations for node removal.