Add additional batch job timestamps

[m-mohr]

Implements #542

Reasoning for the fields based on the discussion in #542 and the existing fields (created, updated, expires):

• created: exists, the time when the batch job was created
• updated: exists, the time of the last status change
• queued: new, the time when the job was queued, to be able to check how long jobs are queued (with started)
• started: new, the time when the job started processing (status changed to running), to be able to check how long processing takes (with updated)
• expires: partially new, the time when the assets get deleted (or if in the past, the time when the assets got deleted) - a field with this description existed in the STAC metadata, added to the job metadata. Would be great if that gets filled if results get deletes so that clients can flag these jobs.
• ended: NOT added, it would always be the same value as updated according to the definition of the updated field.

All properties are optional except for created.

created, updated and expires are somewhat aligned in their meaning with STAC. queued and started don't exist in STAC.

[clausmichele]

An useful addition to track the time a batch job takes without workarounds, thanks! By the way, to add those field to the STAC results we would have to use this extension right? https://github.com/stac-extensions/timestamps/

[m-mohr]

@clausmichele Only for expires, all others are not defined in that extension. Due to slightly different meaning, it could also be omitted.