policy: Add Merge API (#11793)

This commit adds a new API in pkg/bucket/policy package called
Merge to merge multiple policies of a user or a group into one
policy document.

pkg/bucket/policy/principal.go

@@ -90,6 +90,12 @@ func (p *Principal) UnmarshalJSON(data []byte) error {
 	return nil
 }
 
+// Clone clones Principal structure
+func (p Principal) Clone() Principal {
+	return NewPrincipal(p.AWS.ToSlice()...)
+
+}
+
 // NewPrincipal - creates new Principal.
 func NewPrincipal(principals ...string) Principal {
 	return Principal{AWS: set.CreateStringSet(principals...)}