Replace the usage of the shadow-piercing combinator on the ftest framework

We need to replace the usage of the shadow-piercing combinator in the ftest framework, since it was removed on Chrome 69.

As results of this task:

• the shadow adapter was converted into a plugin that contributes new wdio commands, which no longer relies on the shadow-piercing combinator >>>
• because of this, we lost the ability to use xpath selectors, which will be covered in NXP-25871
• added a new test tag @cleanupLocalStorage to make sure that the local storage will be cleaned for the current user after the covered scenario(s) run
• added support for different browsers; you can now use the --browser cli param or the more convenient environment variable BROWSER (i.e., to run the tests with Firefox, you can run BROWSER=firefox npm run test:watch)
• there's a new environment variable BROWSER_BINARY, which allows the path of the browser binary to be changed (applies to Chrome and Firefox only, for now...); this is particularly useful for CI
• updated the Jenkinsfile so these variables will be exposed as job params for the pipeline
• tests are passing on Chrome, but there are still random failures, which will be tackled under NXP-25872
• only 68% of them pass on Firefox, which will be addressed in NXP-25873
• most tests fail on Safari, which we'd like to fix in NXP-25875
• backport for 9.10 will be done separately under NXP-25876