DPLT-1044 Create scheduled Lambda to write lag behind near social

[morgsmccauley]

This PR replaces latest-post-metrics-writer with a scheduled Lambda which writes the block height difference between the QueryAPI social feed and the near social feed. It does this by fetching the latest block height from each API, and writing the difference to CloudWatch.

Writing the metrics from the QueryAPI indexers themselves has the following problems:

• Indexers aren't triggered every block, creating large gaps in the metric history.
• We'll be relying on the QueryAPI infrastructure to tell us everything is ok - is the infra is down we won't be notified.

Having a scheduled Lambda will create a consistent metrics timeline which we can trigger alerts off.