Implement muon.crypto.encryptString/decryptString API

[darkdh]

master key is same as built-in password manager/cookies
Using base64 to encode non ASCII characters

fix brave/browser-laptop#10705

Auditors: @bridiver, @diracdeltas, @bbondy

[darkdh]

@bsclifton mentioned this might cause manually session backup failed because you can only encrypt/decrypt value on the same machine.
What do you think @diracdeltas ?

[darkdh]

saved password and cookies applied to the same rules ^^^
so you can just copy those file to different machine

[diracdeltas]

for safety, i think that muon.crypto.encryptString/decryptString should throw an error or return nothing if OSCrypt EncryptString/DecryptString are not available. In most cases, if a string cannot be encrypted, it is better to save nothing than to save it in plaintext.

[bridiver]

wouldn't it be better to just store them in the password manager? That has the added benefit of cross-device sync either through brave or through the OS encryption storage

[darkdh]

we can do that on Mac and Window because password manager stores encrypted password in Login Data sql file but it stores the login entries in keyring

[diracdeltas]

cc @riastradh for review of the underlying chromium crypto code