Bro plugins should support a patch version (x.y.z)

Since the project and downstream projects such as `bro-pkg` seems to align with semver, plugins should be able to have a MAJOR.MINOR.PATCH version. I suggest adding an optional PATCH version.

A quick review pointed me to the following, but I'm sure there will be some downstream changes and btests to put together:

https://github.com/bro/bro/blob/b99be6458b3fe83284b5262ca0391bf3cc99f7c2/src/plugin/Plugin.h#L66-L82
https://github.com/bro/bro/blob/b99be6458b3fe83284b5262ca0391bf3cc99f7c2/doc/devel/plugins.rst
https://github.com/bro/bro/blob/b99be6458b3fe83284b5262ca0391bf3cc99f7c2/CMakeLists.txt#L61-L65
https://github.com/bro/bro/blob/b99be6458b3fe83284b5262ca0391bf3cc99f7c2/src/plugin/Plugin.cc#L442-L449

My apologies if this is a duplicate, did a quick search and couldn't find it but was on pretty slow Internet at the time.