change: move data epoch offset calculation out of data sources

[AmbientTea]

Description

Removes the data_epoch from the AuthoritySelectionDataSource trait. This method was used internally to apply the 2-epoch offset within each data source method, but was also exposed outside because AriadneInherentDataProvider needed it too in its own logic.

Instead, the data source now simply serves data for the epoch it receives and it's solely AriadneInherentDataProvider's responsibility to calculate it.

This change is backwards compatible.

Checklist

• Commit sequence broadly makes sense and commits have useful messages.
• The size limit of 400 LOC isn't needlessly exceeded
• The PR refers to a JIRA ticket (if one exists)
• New tests are added if needed and existing tests are updated.
• New code is documented and existing documentation is updated.
• Relevant logging and metrics added
• Any changes are noted in the changelog.md for affected crate
• Self-reviewed the diff

[LGLO]

Thank you!