CHI-2997:Refactor search indexing to be notifying

[stephenhand]

Description

• Rename 'reindex' API to be a 'notify' API
• Update the exported functions to use the notification operations rather than the indexing operations
• Update the reindex lambda to handle index operations or notify operations so it can migrate to being fed messages from the SNS topic

Checklist

• Corresponding issue has been opened
• New tests added
• Feature flags / configuration added

Other Related Issues

None

Verification steps

Automated tests
Verify indexing and DW HRM exports still function as expected

AFTER YOU MERGE

1. Cut a release tag using the GitHub workflow. Wait for it to complete and notify in the #aselo-deploys Slack channel.
2. Comment on the ticket with the release tag version AND any additional instructions required to configure an environment to test the changes.
3. Only then move the ticket into the QA column in JIRA

You are responsible for ensuring the above steps are completed. If you move a ticket into QA without advising what version to test, the QA team will assume the latest tag has the changes. If it does not, the following confusion is on you! :-P