[🚀 Feature]: Extensibility for Appium

[p0deje]

Feature and motivation

This is a follow-up to https://github.com/SeleniumHQ/selenium/pull/12141 to implement similar bits across the bindings. In the original PR, this was done for Ruby and the following changes needed to be done:

• Support registering extra headers in HTTP client
• Support overriding User-Agent in HTTP client
• Support registering extra HTTP commands
• Support overriding default locator conversion
• Support registering custom finders for SearchContext
• Support using custom element classes

Likely, the same needs to be done in other bindings.

Usage example

Once bindings extensibility is implemented, we should see Appium using it directly to avoid patching/subclassing the Selenium codebase.

Related issues

• #13949 for Java bindings.