feat: created mocks for tests

[csautter]

The Pull Request #20 introduced integration tests against the real API. As requirement for a successful test run the env var EIA_TOKEN has to be available.
New Pull Requests from fork repositories are not able to access the EIA_TOKEN from the main repo neither the fork. This is a security feature of Github Actions.

• To make the tests independent of an external API call this PR adds mocks of the API responses.
• The tests are functional as independent Unit Tests with mocks or as integration test against the real API (just delete the mocks before test execution)
• The mocks are easily extendible if the tests change
• Github Actions is adjusted to use the mocked data in PRs from fork branches
• a secret scanner was added to avoid accidentally adding request/response mocks with API keys

[csautter]

@philsv i cannot investigate the reason for the failing snyk job (no access)