Expose an API to trigger delta snapshot

[PadmaB]

Story

As a developer, I want to be able to trigger a delta snapshot of etcd, on demand. Triggering via the API should be controlled via a flag

Motivation

The feature will be handy to run certain tests in the CI/CD pipeline, not having to wait for the scheduled time. Controlling the schedule for triggering the delta snapshot is not always an option.

Acceptance Criteria

• Should be able to trigger an on-demand delta snapshot

Definition of Done

• Knowledge is distributed: Have you spread your knowledge in pair programming/code review?
• Unit tests are provided: Have you written automated unit tests?
• Integration tests are provided: Have you written automated integration tests?
• Minimum API exposure: If you have added/changed public API, was it really necessary/is it minimal?
• Operations guide: Have you updated the operations guide about ops-relevant changes?
• User documentation: Have you updated the READMEs/docs/how-tos about user-relevant changes?