Add a `jsp/web_shell` payload

[postmodern]

Add a JSP payload which executes a command and returns the output upon request. It should be able to generate a compiled JAR or a WAR file.

Examples

• https://github.com/jbarcia/Web-Shells/tree/master/laudanum/jsp/warfiles
• https://github.com/LaiKash/JSP-Reverse-and-Web-Shell/blob/30ae24510b81f2825779ae7551d28ec57d057fc4/shell.jsp#L59-L102