Save extension settings into project file

[aph3rson]

Previously, Burp did not allow for easy state tracking for extensions across restarts of Burp/reloads of the extension/reloads of the project. This meant that extensions had to either be reconfigured across every reload, or do hacky things to track extension state as a bogus request in the sitemap.

However, Burp now supports the saveExtensionSetting and loadExtensionSetting API methods, which allows saving simple key/value pairs associated with the extension. These settings are durable across extension reloads, project reloads, and Burp restarts.

It'd be great if AWSSigner supported these methods, and saved extension state into the project file itself.