[Enhancement] Consistent handling of cookies across platforms

## Summary
Cookies are persisted across app sessions on Android whereas on iOS they're not. It would be ideal to have control over this behavior regardless of any inconsistencies that may exist between the underlying platform components.

## API Changes

Provide the ability to configure the cookie persistence behavior from Xamarin.Forms.

e.g.

WebView.CookiePersistence = CookiePersistence.None; // new API

Options could indicatively include:
- None
- SingleSession
- MultiSession

## Intended Use Case
Having more granular control over cookie persistence without creating a custom WebView.

Start by reviewing the Xamarin.Forms WebView API and how Android and iOS currently persist cookies. Define the supported persistence options and how they should behave across platforms, then verify that callers can configure the behavior without creating a custom WebView.

Written by the indexing model from the issue text.