Add support for loading and unloading web extension in webdriver

[kiaraarose]

This is a pre-requisite to adding support for cross browser testing for web extensions. PR to test these new commands in WPT: web-platform-tests/wpt#49786

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[OrKoN]

This functionality is already specified as part of WebDriver BiDi. Is there additional context for this PR? Unless there is a good reason, it does not sound like WebDriver specs should specify this twice.

[davidjohnson91]

@OrKoN Safari does not support WebDriver BiDi and we don't want that to prevent us from adding WebExtension testing support.

[OrKoN]

Is it for testing within WPT?

[xeenon]

@OrKoN Yes, see web-platform-tests/wpt#49786.

[OrKoN]

Thanks, the linked PR seems to be testing the WebDriver implementation defined by this PR. If the new endpoints are primarily meant to be used to test browser implementations of web extensions in WPT, testdriver.js allows implementing commands that match the WebDriver BiDi specification (the implementation can be vendor-specific and does not require WebDriver BiDi support).