WebDriver: install command optional parameters

[chrmod]

Current specs for WebDriver BiDi and proposal for WebDriver Classic both provide a basic capability to install a web extension.

But each browser may support additional parameters, for example to:

• Allow in Incognito/Private Window/InPrivate/Private Browsing (Chromium/Firefox/Safari)
• Allow access to file URLs (Chromium)
• Collect errors (Chromium)
• Install temporarily (Firefox)
• Allow automatic updates (Firefox)
• Allow access to search page results (Opera)
• Pin to toolbar (Opera)

Are there any other params that a WedDriver should support?

How can we allow optional browser specific parameters?

[chrmod]

Related to #19 (comment) and #760

[kiaraarose]

This is something Safari would be supportive of, especially in the case of granting permissions for the extension. In our sync meetings for cross browser testing, we decided to grant all of the permissions defined in the manifest automatically, but we will definitely have tests where we'd want to test that extension doesn't have permission if it wasn't granted by the user. So adding a parameter to disable the auto grant when the extension is loaded would be nice.

[xeenon]

Install temporarily (Firefox)
Allow automatic updates (Firefox)

All installs should be temporary when using web driver — this should not be a vector for malware to install malicious extensions. (So these should not be an option.)

Any option that isn't supported by one browser can just be ignored by the other browsers or trigger an error if that makes sense. But we should agree on any common options.