Foundation: Port ImapClient 3 to PSR-4 and PHP8.1+ interfaces

[ralflang]

• consistently use Horde Streams instead of raw stream handles
• use union and intersection types instead of classes which try to be two things at once
• stream-first interface, to-string on demand rather than conditional return type
• Config class rather than options array
• PSR-14 EventListener interface for observability
• Horde Stream for imap connection dump instead of "debug" option
• PSR-16 SimpleCache interface for caching concerns (probably implement PSR-6 interface later as a better fit)
• All optional collaborators default to Noop/Null implementations rather than "null" to avoid need to check on every step
• lazy implementation, i.e. only connect once we really need to. No connection on construct or deserialization
• Feature parity with old implementation except "deprecated"
• Keep lib/ as-is for now but will deprecate in Horde 6 lifecycle and abandon later
• Keep all attribution to @slusarz and other original authors - we didn't invent this thing, we just repackage it

Prerequisite for #39