Feature and motivation

@nickgaya added extensions in common directory https://github.com/seleniumhq/selenium/tree/trunk/common/extensions

Would be nice for all languages to update to use these extensions in their test code.
Usage example here: #10216

Usage example

add to bazel file as data:

data = TEST_DATA + [
    "//common/extensions",
    ...

then can reference it from main directory:

/common/extensions/webextensions-selenium-example.crx