Add AkonadiAddressbook plugin

Akonadi is centralized database to store, index and retrieve the user's
contacts and more since KDE SC 4.4. This plugin adds support for
completion and pretty names.

Once upon a time there was an Akonadi plugin but has been removed due
to KDE4/Qt4 dependencies [1]. This plugin is pure KF5 port.

[1] See 5f3f05ef and 5eb88fad

CCBUG:358485
Change-Id: I5dcb128d8fa9012561096361fbdbe0ff19493e29