Provide classpath instead of jar when invoking java

[doublemarket]

When starting a Selenium process in a container, the startup script invokes the java command with the -jar option. However, it's not possible to add classpaths when running a Selenium process even if we use the JAVA_OPTS because when the -jar option is used, the -cp option which provides classpaths is ignored. If we want to use a custom proxy or an add-on as a Jar file without changing a container, -cp is needed.

This pull request makes a change to the startup scripts to use -cp instead of -jar and introduce the JAVA_CLASSPATH environment variable to overwrite the default classpaths.

This shouldn't break existing usages without providing the JAVA_CLASSPATH environment variable.

Do you think this is valuable to have? If so, I'll update the README file as well.

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[diemol]

Thanks for the PR @doublemarket!
I think it makes sense, it simplyfies how people can add their own generated jars.
We can merge this PR when a proper documentation on how to use this env var is added to the PR.

[doublemarket]

@diemol Thanks for reviewing and I've updated the README file with an example!

[diemol]

Thank you @doublemarket!