Make it possible to import private key during setup

Currently it is possible to import the private key after setup (sans some problems in DeltaChat fails to import keys), but only after generating a new key. As a result, this key cannot be used to decrypt old messages.

This feature proposal is about adding an option to select the key during setup in advanced settings. This can also be a place for the generated key type option requested in Allow RSA 2048 and 4096 bit keys, a single setting with multiple options:

  1. Generate ECC key (default)
  2. Generate RSA key
  3. Import existing key

Currently fetch_existing_msgs setting is disabled (fix: disable `fetch_existing_msgs` setting by default by link2xt · Pull Request #4423 · deltachat/deltachat-core-rust · GitHub) but we may consider enabling it again as an option, e.g. if the user has the key backed up and wants to get old messages from the server.