Add environment variables for SendDataOnExit functionality

[JcolemanNR]

Description

This PR adds new environment variables to control SendDataOnExit functionality:

• NEW_RELIC_SEND_DATA_ON_EXIT: (boolean) enables/disables send data on exit
• NEW_RELIC_SEND_DATA_ON_EXIT_THRESHOLD_MS: (uint) sets the minimum number of MS that the application must have been running before data will be sent on exit (when enabled).

Includes unit tests.

Author Checklist

• Unit tests, Integration tests, and Unbounded tests completed
• Performance testing completed with satisfactory results (if required)
• Agent Changelog or Lambda Agent changelog updated

Reviewer Checklist

• Perform code review
• Pull request was adequately tested (new/existing tests, performance tests)
• Review Changelog

[nr-ahemsath]

Changes look good. Glad to see we're increasing customers' ability to configure the agent by using environment variables, as this is often the most convenient way in cloud-hosted and/or containerized scenarios.