Handle contact events with date/time and time zone offset (#25)

* Added birthday with timezone test

Signed-off-by: Arnau Mora <arnyminerz@proton.me>

* Added test for `ZonedDateTime`

Signed-off-by: Arnau Mora <arnyminerz@proton.me>

* Add comment, remove println

* Updated test name and assert

Signed-off-by: Arnau Mora <arnyminerz@proton.me>

* Test offset to UTC conversion, too

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Signed-off-by: Arnau Mora <arnyminerz@proton.me>
Co-authored-by: Ricki Hirner <hirner@bitfire.at>