Make change to robots.txt configurable

[raffaelj]

As described in #282, robots.txt produces invalid output. I don't need that change to robots.txt at all, because I deny direct access to cached files via .htaccess:

# Deny direct access to cache folder, but allow internal redirect
RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteRule ^wp-content/cache/cachify/ - [F]

Expected behavior

I want to click a checkbox in the settings ui to disable automatic changes to robots.txt. When disabled, the hook do_robots is skipped.

System:

• test docker instance with apache, php81
• WP 6.3
• Cachify dev branch, hdd mode