Proxmox inventory filters (#4352)

* Proxmox inventory plugin - Initial implementation of filters

  * This is an attempt at implementing something that would satisfy
    issue #3553
  * A rather massive code rewrite was needed as adding the host to the
    inventory, setting its variables and adding it to various groups
    used to be done as soon as the information became available. This is
    not possible when it is not known whether the host should be added
    to the inventory before all data has been gathered.
  * The code for both LXC containers and Qemu VMs was refactored into a
    single loop.
  * Helper functions to generate group and fact names were added.

* Proxmox inventory plugin - Warnings for filter errors

  * When an error occurs while compositing a filter's value and strict
    mode is disabled, display a warning.

* Proxmox inventory plugin - Fixed pool groups building

  * Hosts that were excluded by the host filters were still being added
    to pool groups, causing errors.

* Proxmox inventory plugin - Refactoring

  * Split off the VM/container handling code from the
    `_populate()` method
  * Split off pool group attribution from the `_populate()` method

* Proxmox inventory filters - Changelog fragment

* proxmox inventory - Simplify _can_add_host() method

Co-authored-by: Felix Fontein <felix@fontein.de>

Co-authored-by: Felix Fontein <felix@fontein.de>

changelogs/fragments/4352-proxmox-inventory-filters.yml

@@ -0,0 +1,4 @@
+---
+minor_changes:
+  - proxmox inventory plugin - add support for client-side jinja filters
+    (https://github.com/ansible-collections/community.general/issues/3553).