nmcli: avoid changed status for most cases with VPN connections (#5126)

* nmcli: avoid changed status for most cases with VPN connections

Follow-up https://github.com/ansible-collections/community.general/pull/4746

* `nmcli connection show` includes vpn.service-type but not vpn-type.
  Switching to vpn.service-type removes unneeded diffs while keeping
  the same functionality, as vpn-type is an alias of vpn.service-type
  per nm-settings-nmcli(1).

  NetworkManager also adds `org.freedesktop.NetworkManager.` prefix for
  known VPN types [1]. The logic is non-trivial so I didn't implement it
  in this commit. If a user specifies `service-type: l2tp`, changed will
  be always be True:

    -    "vpn.service-type": "org.freedesktop.NetworkManager.l2tp"
    +    "vpn.service-type": "l2tp"

* The vpn.data field from `nmcli connection show` is sorted by keys and
  there are spaces around equal signs. I added codes for parsing such
  data.

Tests are also updated to match outputs of nmcli commands.

[1] https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/1.38.4/src/libnm-core-impl/nm-vpn-plugin-info.c#L619

* Add changelog

* Some suggested changes

* Make space stripping more flexible - works for cases without equal
  signs.
* Keep vpn.data in a test case with no spaces

* nmcli: allow any string for vpn service-type

changelogs/fragments/5126-nmcli-remove-diffs.yml

@@ -0,0 +1,2 @@
+bugfixes:
+  - "nmcli - avoid changed status for most cases with VPN connections (https://github.com/ansible-collections/community.general/pull/5126)."