Handle extension capabilities in the desired capabilities flow

[mdmintz]

Handle extension capabilities in the desired capabilities flow. These are formatted as string:string, and is the pattern for passing extra vendor specific capabilities. See https://www.w3.org/TR/webdriver1/#dfn-extension-capability

[mdmintz]

Work completed by @TaylorMcGinnis and merged in #1452
(Will be part of the next release.)

[mdmintz]

@TaylorMcGinnis This has been released in version 4.0.0 - https://github.com/seleniumbase/SeleniumBase/releases/tag/v4.0.0