Add method to change the key(passcode) on an existing encrypted database about encrypted-core-data

[gavin-black]

This will probably not happen directly, since I would think any password change mechanism would be at a higher level, and it in turn would be managing keys. I'm very hesitant to couple ECD with any extra dependencies by itself.

For example you could use secure-foundation (https://github.com/project-imas/securefoundation) to store your DB key encrypted in the keychain, and then replace that keychain entry later. But when and how you update your keychain would be dependent on the mechanism for updating your password.

Right now as a test we are adding this exact capability to ReMail (https://github.com/project-imas/Remail-iPhone) and will likely integrate it later into securefoundation or app-password.

Closing for now, but will update if/when we add that capability to other parts of iMAS.