Add proxy settings to browser context options

[wizrds]

This pull request exposes the ProxySettings struct to the BrowserContextOptions to support specifying proxy options when creating a new context.

[padamson]

Thanks for this contribution! The implementation looks good and matches the Playwright API.

A few changes I'll make before merging:

1. Breaking change: The removal of ProxySettings from api::launch_options could break existing users--adding a re-export in api/mod.rs for backward compatibility.

2. Tests: Adding a basic test to verify the proxy option serializes correctly

Otherwise, this looks ready to merge