Enable curl e2e backend via common interfaces and add HTTP backend to integration

[yagikota]

What would you like to be added?

• Implement the common interfaces for the e2e curl backend

  • Make tests/framework/e2e/curl.go conform to tests/framework/interfaces/interface.go so tests/common/ suites can run with the curl backend in e2e.

• Implement the common interfaces for the integration HTTP backend

  • Provide an HTTP backend in the integration test framework that implements the same common interfaces as the existing gRPC backend.

• Allow tests/common/ suites to run against both gRPC and HTTP backends in integration.

Why is this needed?

As discussed in #13637 (comment), e2e provides both etcdctl and curl backends, but only etcdctl implements the common interfaces. This means tests/common/ cannot run over curl in e2e.
On the integration side, there is only a gRPC backend, so we cannot run tests over curl in tests/common/.

Enabling the e2e curl backend through the common interfaces and adding the HTTP backend to integration will allow us to run common tests against the curl(e2e)/HTTP(integration) backend.

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Goals

E2E

• Extend tests/framework/e2e/curl.go to implement the common interfaces in tests/framework/interfaces/interface.go.

Integration

• Introduce an HTTP backend (e.g., tests/framework/integration/http.go) implementing the same common interfaces as the existing gRPC backend.

Harness

• Allow tests/common/ to run with:

  • e2e: E2E_BACKEND=etcdctl|curl (default: etcdctl)
  • integration: INTEGRATION_BACKEND=grpc|http (default: grpc)

After providing two backends for each test, we need to provide backend selection (via environment variables), so we can easily switch the backend.

Document

• Write tests/README.md to explain e2e/integration tests as well as the common test framework.

  • Example: https://github.com/yagikota/etcd/blob/main/tests/robustness/README.md

Non-goals

• Migrating the existing e2e/integration tests to the common framework (tracked separately in Unify testing framework #13637).
• Reducing the e2e test matrix (tracked separately in Reduce the time e2e tests take #18983).

[yagikota]

cc @serathius @ronaldngounou