code: public CRestBuilder placeholder(String placeholder, String value) {
          return addPlaceholders(singletonMap(placeholder, value));
      }
docs: <p>Adds the given placeholder to the string-based annotations placeholders replacement map.</p>
  <p>So for the given annotation:</p>
  <code><pre>
  &#64;EndPoint(&quot;http://{app.host}:{app.port}&quot;)
  </pre></code>
  <p>The placeholders will be replace if the builder is configured as follow:</p>
  <code><pre>
  String server = ...;
  String port = ...;
  CRestBuilder builder = ...;
  
  builder.placeholder("app.host", server)
  .placeholder("app.port", port);
  </pre></code>
  @param placeholder the placeholder to be replaced
  @param value the value to replace the placeholder with in string-based annotations
  @return current builder