Create a PSR-4 MailboxAccountManager

[ralflang]

Create a modern MailboxAccountManager abstraction:

• PSR-4
• Proper DI
• No intrinsic distinction between a "primary" and "remote" mailboxes
• Pluggable provider model (the former "primary" mailbox is just one of many possible providers, the "user prefs" provider is another one)
• Capable of having a provider coopt mailboxes from OIDC/OAuth2 claims
• unit testable
• Support at least the PSR-4 variants of horde/imap_client pop3 and imap boxes
• Open to extension, i.e. support potential other mailbox types such as rss/atom, jonah feed subscriptions or provided by other horde registry apps