Implement caching of the user-agent for headless UC Mode

[mdmintz]

Implement caching of the user-agent for headless UC Mode

First the explanation of why this is needed:

The last release fixed a bug (#2106) where headless UC Mode stopped working on Chrome 117 because HeadlessChrome appeared in the user-agent string. This situation was fixed by first opening a regular headless browser, getting the user-agent string, replacing HeadlessChrome with just Chrome, and then opening a UC Mode browser with the modified user-agent string.

This led to opening more browsers than needed. Eg. If a pytest suite had 20 tests for headless UC Mode, that meant that each of those tests opened an extra browser for the user agent string, leading to 40 browsers opened. That action should only be needed once per suite run (probably much less often than that, but cannot predict when Chrome will be updated). So for 20 headless UC Mode tests, only 21 browsers should be needed (with the first one grabbing the user-agent string to be modified for the others).

[mdmintz]

This was resolved in 4.18.7 - https://github.com/seleniumbase/SeleniumBase/releases/tag/v4.18.7