Support snapshots configurations in stats collector, fix bugs where snapshot retention didn't support past DAY format, fix false positive edge case in snapshot event

[Will-Lo]

Summary

Issue Briefly discuss the summary of the changes made in this
pull request in 2-3 lines.

We want to be able to emit events with user tables that have the history configuration for them so that monitoring systems can accurately detect when snapshot expiration fails for configured tables.
This PR also does some refactoring around parsing policy JSONs to be more robust and easier to manage as more policies are added.

This PR also fixes a bug where not all date granularities were supported in retention, since TimeUnit is maxed granularity at DAYS when technically the snapshot policy can be kept at higher granularities of MONTH and YEAR, although we currently still limit it to 3 DAYS.

We also add the secondOldestSnapshot field to the IcebergTableStats to handle the edge case where a table has very infrequent writes, this can cause snapshots to fall out of retention policy but not yet be cleaned up, if the secondOldestSnapshot is still recent then we should avoid being alerted.

Changes

• Client-facing API Changes
• Internal API Changes
• Bug Fixes
• New Features
• Performance Improvements
• Code Style
• Refactoring
• Documentation
• Tests

For all the boxes checked, please include additional details of the changes made in this pull request.

Testing Done

• Manually Tested on local docker setup. Please include commands ran, and their output.
• Added new tests for the changes made.
• Updated existing tests to reflect the changes made.
• No tests added or updated. Please explain why. If unsure, please feel free to ask for help.
• Some other form of testing like staging or soak time in production. Please explain.

For all the boxes checked, include a detailed description of the testing done for the changes made in this pull request.

Unit tests, tested snapshot version and ran tablestatscollector job in a cluster on Spark, did not see any errors.

Additional Information

• Breaking Changes
• Deprecations
• Large PR broken into smaller PRs, and PR plan linked in the description.

For all the boxes checked, include additional details of the changes made in this pull request.

[chenselena]

very clean refactor Will, thanks
left a couple of questions regarding the old code

[cbb330]

excellent, thanks for adding secondOldestSnapshot which will solve frequent false positives when monitoring the availability of Snapshot Expiration job.