feat: allow changing to a shared drive

[LouisBrunner]

For my use-case, I need to be able to change the current share that ProtonDrive points to.

Luckily MainShare, MainShareKR and RootLink are exposed so I can change those. However, if I am trying to switch to a folder that has been created and shared by another account, it fails. This is because the creator's keyring is not available in addrKRs/addrData.

As I couldn't find a way to get the proton.Address for another account, I added a new map next to the existing ones which goes from email to keyring without going through the addr.ID.

Moreover, I needed to be able to get a keyring from addrKRs so I added a getter for that.

I am not sure if this is the best solution or if you would want to do the switching directly in this package. You can see my usage here: https://github.com/LouisBrunner/gopy-ha-proton-drive/blob/main/go/share.go#L55-L125

Thank you for this great package by the way! 😄