Add PORT_VERSION cmake variable to be used in portfile.cmake

[CJCombrink]

---

Is your feature request related to a problem? Please describe.
Currently the maintainers need to ensure that the version specified in the CONTROL file matches the version referenced in the portfile.cmake. This is not ideal since these versions can get out of sync which can be problematic if a user sees that a port is at version x but the installation installs version y. See for example #9235

Proposed solution
Add a PORT_VERSION variable to cmake that will be available to the portfile that contains the version specified in the CONTROL file.

Describe alternatives you've considered
I have proposed on PR the above to manually read/parse the CONTROL file for the given port in the portfile but this is not ideal.

file(STRINGS "${CURRENT_PORT_DIR}/CONTROL" PORT_VERSION REGEX "Version: .+")
string(REGEX REPLACE  "Version: (.+)" "\\1"  PORT_VERSION "${PORT_VERSION}")

Additional context
I will make a PR shortly

[CJCombrink]

Please review my draft PR.
I have almost no experience with cmake thus have no idea if there are better ways to do what I am doing, any help/support/suggestions will be appreciated.