Add e2e tests to CI pipeline

E2E tests were added [here](https://github.com/babylonlabs-io/staking-api-service/pull/290), but they are not part of CI yet. 

We need a way to trigger **only** e2e tests (currently `make test-e2e` will run unit tests as well). Also total coverage should be as a result of CI runs (e2e + unit-tests + integration tests).