chore: Avoid sending unnecessary requests if corresponding extensions are absent

[mykola-mokhnach]

Change list

As @KazuCocoa mentioned in appium/python-client#856 (comment) the change could slow down user scripts, especially if the server runs in cloud env, and we send two requests instead of one (e.g. the server is running an older driver version which does not support the given mobile endpoint yet).
In such case we want to remember if the given API is actually supported by the given driver instance, so next time we don't need to call the server twice, and thus save some time on each request.

Types of changes

• No changes in production code.
• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)