Bot.java: add support for "commandPrefixRequired" toggle [JENKINS-17380, JENKINS-58927]

[jimklimov]

As posted in https://issues.jenkins.io/browse/JENKINS-17380 and https://issues.jenkins.io/browse/JENKINS-58927 currently the IRC Bot allows interaction via private chats (admin to bot) as opposed to group channels; however this still requires using the command prefix or nickname prefix - which is cumbersome in an already individualized private chat.

The culprit was in the common instant-messaging-plugin, where it recognizes if the message is of interest to the bot, by requiring those prefixes.

A viable solution (also backwards-compatible for users who would continue to use the prefixes) is to have an option to not-require the prefix and just try using any received message, if the consumer plugin told this bot instance to do so (not default!)

After merging and releasing a plugin iteration with this change, there would be fixes applied to ircbot-plugin to use the new constructor API. => jenkinsci/ircbot-plugin#214

CC @Flowdalic : You may want to update jabber-plugin similarly?

Testing done

Currently dev-tested for API non-breakage using a local mvn install and using the snapshot version as the required dependency version in ircbot-plugin build area (where the consumer is updated to use the new toggle). IDEA recognizes both of the constructors.

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[jimklimov]

Auto-Released as https://github.com/jenkinsci/instant-messaging-plugin/releases/tag/2.768.v82b_9ec69388f