feat!: declare the guest agent per guest OS, and share what provisioning needs

GUEST_AGENT_PACKAGES / GUEST_AGENT_RUNCMD described one agent per
hypervisor. A hypervisor that provisions FreeBSD and OpenBSD guests needs
a different agent per guest, and netOrk has to know which guests a driver
can provision at all (NetOrk/netork#794). Both now come from one
class-level mapping netOrk reads off the driver class:

    GUEST_AGENTS = {guest_os: (packages, runcmd)}

Its keys are the guests the driver provisions, and
create_vm_from_cloud_init(guest_os=...) refuses any other. The base keeps
Linux with qemu-guest-agent.

New module `provisioning`, generic for every hypervisor:
- QEMU_GUEST_AGENTS: the QEMU agent for Linux, FreeBSD and OpenBSD, with
  package names and service commands verified on FreeBSD 15.1 and
  OpenBSD 7.9 cloud images (NetOrk/netork#793).
- network_config(): cloud-init's network-config v2 with MAC matching,
  moved here from napalm-vmware. FreeBSD's nuageinit reads no other
  version.
- split_compression(): how a packed image (.xz, .gz, .bz2, .zst) is
  unpacked.

BREAKING CHANGE: GUEST_AGENT_PACKAGES and GUEST_AGENT_RUNCMD are gone;
drivers declare GUEST_AGENTS instead.

README.md

@@ -312,6 +312,16 @@ class ProxmoxDriver(HypervisorDriver):
         ...
 ```
 
+A hypervisor that provisions VMs from cloud images declares, per guest OS,
+the agent cloud-init installs so the hypervisor can read the new VM's IP:
+`GUEST_AGENTS = {guest_os: (packages, runcmd)}`. Its keys are the guests the
+driver can provision, and `create_vm_from_cloud_init(guest_os=...)` refuses
+any other. A QEMU-based driver takes `provisioning.QEMU_GUEST_AGENTS`
+(Linux, FreeBSD, OpenBSD). `provisioning.network_config()` writes the
+network-config v2 every cloud-init flavour reads (FreeBSD's nuageinit reads
+no other), and `provisioning.split_compression()` says how a packed image is
+unpacked; both are the same for every hypervisor.
+
 ### OS / Linux
 
 ```python