Generalize `AnyError`

[schreter]

Currently, openraft uses anyerror crate, which allows passing "any" errors.

However, AnyError is understandably somewhat limited, since it was written for general public. In our project, we use a different version of AnyError which allows better diagnostics and also stores some errors in-place. Plus, it won't panic on OOM.

Therefore, it would be helpful to generalize AnyError via the configuration to be able to use a different implementation. Since the API is quite small, it should be fairly trivial.

[github-actions]

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