mbsync: extend config type with list of strings

Some configuration options can take space separated strings; for
example `SSLVersions` can be configured with multiple allowed
versions.

    SSLVersions TLSv1.3 TLSv1.2

This can now be represented in Home Manager.

    SSLVersions = [ “TLSv1.3” “TLSv1.2” ];

In implementing this change, it uses oneOf for config type, as it is a
cleaner way to represent the union than the nested eithers
formulation.

Also add SSLVersions to test lists of strings in
`account.extraConfig`.

modules/programs/mbsync-accounts.nix

@@ -4,7 +4,8 @@ with lib;
 
 let
 
-  extraConfigType = with lib.types; attrsOf (either (either str int) bool);
+  extraConfigType = with lib.types;
+    attrsOf (oneOf [ str int bool (listOf str) ]);
 
   perAccountGroups = { name, config, ... }: {
     options = {