Update browser wallet to protect private keys

[OR13]

https://github.com/decentralized-identity/ua-web-extension/blob/master/global/js/web-keystore.js#L111

For signatures that can just use window.crypto, we should protect them from extraction.

Keys that are needed for on page signing can't use this obviously, so secp256k1 is probably not going to work.

[OR13]

However, this could be very useful for P2P UA connections for PWAs.

Imagine you have 2 wallet apps, which are managing keys for DIDs.

The 2 apps configure a serviceEndpoint to rendevouz and add non extractable keys to their respective DID documents.

After the DID Update operations succeed, an end to end encrypted p2p connection can be established between the 2 wallet apps.

The 2 apps can privately communicate over webrtc, with confidence that the decryption keys cannot be extracted (easily) from the browser.