Add --properties-separator option (#1951)

This commit adds the --properties-separator option, which lets users
specify the separator used between keys and values in the properties
output format. This is done by adjusting the value of
github.com/magiconair/properties#Properties.WriteSeparator at encode
time.

Some refactoring of the properties encoder unit tests was done to make
it easier to write unit tests that include different separator values.

Fixes: #1864

Signed-off-by: Ryan Drew <ryan.drew@isovalent.com>

pkg/yqlib/properties.go

@@ -0,0 +1,13 @@
+package yqlib
+
+type PropertiesPreferences struct {
+	KeyValueSeparator string
+}
+
+func NewDefaultPropertiesPreferences() PropertiesPreferences {
+	return PropertiesPreferences{
+		KeyValueSeparator: " = ",
+	}
+}
+
+var ConfiguredPropertiesPreferences = NewDefaultPropertiesPreferences()