Add a cmd/hex_escape encoder

[postmodern]

Add a built-in command payload encoder that converts each argument to a shell hex string using Support::Encoders::Shell.encode(string).

• ls -l -> bash -c "$'\x6c\x73' $'\x2d\x6c'"

Use Shellwords.shellsplit to split the command into arguments.

[postmodern]

Confirmed $'\x6c\x73' $'\x2d\x6c' works in zsh and mksh.

[postmodern]

There shouldn't be cmd/ encoders, since CommandPayloads can represent either UNIX shell commands or Windows (cmd.exe or PowerShell) commands.