[Push] Support UnifiedPush Connector 3.x

[rfc2822]

Scope

• I'm sure that this feature request belongs here and not into Discussions.

Description

There's a new UnifiedPush connector major version 3.x, which seems to add encryption.

A few things have to be changed on our side to update to 3.x.

Further info

No response

Depends on bitfireAT/dav4jvm#63

[github-actions]

This PR/issue depends on:

• Incorporate latest changes from WebDAV-Push dav4jvm#63