Rename new Hyper-V driver to "Host Compute System (HCS)"

Given that the upcoming release of Multipass will introduce a new driver on Windows that will eventually replace the existing "Hyper-V" driver, and with both drivers being available at the same time during at least one release cycle, the new driver needs to be named such that it is easily distinguishable from the old/existing driver.

1. For any user facing appearance, the term "Host Compute System (HCS)" must be used. This applies to CLI, GUI, and documentation.
2. For the settings value, `hcs` should be used.
3. Class names and other code elements should not be renamed as the existing "Hyper-V" driver will be removed eventually.