Implement support for bucket object retention plans

[beikov]

The idea is to introduce something like a bucket object retention plan which can be assigned to bucket objects. If needed we could introduce a default assignment to objects based on e.g. bucket object prefix.

The plan defines a retention period e.g. 30 days and an action. Interesting actions are

• Move to storage + set new retention plan
• Delete
• Publish a bucket object event

The use case is to provide a temporary storage for e.g. attachments that are used when sending an email. An attachment can be put into the bucket with a retention of e.g. 30 days. An e-mail is sent with the bucket objects as attachment. The bucket object is kept around for a while in case it is necessary to debug or re-send the e-mail.