lib/cli: add wrappingValueString option

The function currently doesn't provide a way to create a result like
`--foo "bar"` or `--foo="bar"`, where the argument is quoted. This
option resolves that. We set it to an empty string by default, so we can
always interpolate it into the string, even if the user didn't set it.

lib/cli.nix

@@ -84,6 +84,13 @@ rec {
     By default, the separator is set to a space, so option `-c` and value `5` would become ["-c 5"].
     This is useful if the command requires equals, for example, `-c=5`.
 
+    `wrappingValueString`
+
+    : The string to surround an option's value with.
+    Values aren't quoted by default, so option `foo` and value `bar` would become ["--foo bar"]
+    This is useful if your value could be misinterpreted by your shell without quoting.
+    For example, if you wanted a result of `["--foo \"bar\" "]`, you could set `wrappingValueString` to a literal quote.
+
     # Examples
     :::{.example}
     ## `lib.cli.toGNUCommandLine` usage example
@@ -120,6 +127,8 @@ rec {
 
       optionValueSeparator ? " ", # Space by default, reflecting `--foo bar`
 
+      wrappingValueString ? "",
+
       mkOption ?
         k: v:
         if v == null then
@@ -129,7 +138,9 @@ rec {
             (lib.concatStrings [
               (mkOptionName k)
               optionValueSeparator
+              wrappingValueString
               (lib.generators.mkValueStringDefault { } v)
+              wrappingValueString
             ])
           ],
     }: