Support `SentryEnvelopeHeader#sent_at`

### Description

Context https://github.com/getsentry/team-mobile/issues/32
We have to check if this field has to be generated during the event creation or sending (thru Native SDKs).
Maybe requires adding this field to Native SDKs as well.