Add Anexia cloud provider (#842)

* implement basic anexia cloud provider

the commits introduce basic implementation of anexia
cloud provider, the provider does not support MigrateUID

* anexia cloud provider refactoring

the commit adjusts code style to existing implemntations and
adds additional error checks

Signed-off-by: Marcin Franczyk <marcin0franczyk@gmail.com>

* upgrade go-anxcloud dependency to v0.2.0

* manage ip allocations

store ip allocation identifier on machine creation in the providerStatus field
and free ip on machine deletion

* add integration tests for anexia cloud provider

Signed-off-by: Marcin Franczyk <marcin0franczyk@gmail.com>

* clean up minor things in the anexia provider

- correctly sort import groups
- remove unused Config.SSHKey field
- fix copy-paste comment

Co-authored-by: Marcin Franczyk <marcin0franczyk@gmail.com>