fix: allow embedded newlines in arguments

[sjinks]

This PR updates validateCommand() to allow for whitespace characters in the arguments.

validateCommand() used to merge consecutive whitespace characters and replace them with the space character. This broke things when we needed to pass multiple whitespace characters (e.g., preformatted text) or embed a newline character.

Test:

go build && ./cron-control-runner -token xxx

$ echo -e "xxx;$(uuidgen);53;206;option  set \n xxx \"a\nb\"" | nc 127.0.0.1 22122
Success: Updated 'xxx' option.

$ wp option get xxx
a
b

⚠️ the base branch for this PR is BB8-12231 (#32) because it relies upon the new logic in tokenizeString introduced in that PR.