adds a backoff strategy to the OutboxProcessor for retrying failed 
events and updates the query logic for selecting unpublished events. 
renames retry logic to utilize `next_retry_at` for error handling, 
ensures compatibility with existing code by maintaining the deprecated 
`retryBackoff` field. includes comprehensive tests for backoff 
behavior and validates the processing of outbox entries.

feat(outbox): pluggable backoff strategy with exponential default

See merge request !299

chore(release): prepare for v1.20.0

See merge request !300