Add initial templating support to Vault credential libraries (#2569) (#2575)

Add initial templating support to Vault credential libraries

This adds support for templating various account and user values into
Vault dynamic credential library POST bodies and GET/POST paths.

It also unifies how parameters are used in grants, with backwards
compatibility.

internal/util/pointer.go

@@ -0,0 +1,8 @@
+package util
+
+// Pointer is just a generic function to return a pointer of whatever type is
+// given
+func Pointer[T any](input T) *T {
+	ret := input
+	return &ret
+}