Added immutable option to fsattrs body

This introduces a new fsattrs immutable body for use with the files promise
type. When applied, it ensures that the promised file has the immutable (i)
attribute set. The agent will transparently manage the attribute, removing it
before any necessary file modifications and restoring it afterwards.

Ticket: CFE-4582
Changelog: Title

Author: nickanderson

lib/files.cf

@@ -1936,6 +1936,34 @@ body delete tidy
       rmdirs   => "true";
 }
 
+@if minimum_version(3.27)
+##-------------------------------------------------------
+## fsattrs
+##-------------------------------------------------------
+
+body fsattrs immutable
+# @brief Ensure promised file is immutable
+#
+# **Example:**
+#
+# ```cf3
+# files:
+#   "/etc/motd"
+#     content => "There are, in fact, rules. You have been notified.",
+#     fsatters => immutable,
+#     perms => mog( "644", "root", "root" );
+# ````
+{
+      # When true the end state of the file is immutable. If the promise
+      # necessitates a change in the file, the agent will handle immutability
+      # transparently with the bit being removed just before modification
+      # and restored immediately after the atomic change is made.
+
+        immutable => "true"; # The i attribute supported by chattr
+}
+
+@endif
+
 ##-------------------------------------------------------
 ## rename
 ##-------------------------------------------------------