As an organizer I want to claim expenses that need to be recorded in the Incomes Register

In order to pay kilometer based travel expenses for using an organizer's own car, those expenses need to be recorded in the Incomes Register. In order to record an expense in the Incomes Register, we need the social security number of the organizer. We would prefer not to store social security numbers unencrypted.

In order to support this use case, we would like to have a field type that is encrypted asymmetrically and can be decrypted only by the intended recipient. See #380.

[Blueprint](https://outline.con2.fi/doc/taloushallinto-6UhhJreExZ)
[Discussion on Slack](https://con2.slack.com/archives/C3ZGNGY48/p1705953140325369)