feat: Tests structure

[kukkok3]

Description

Some groundwork for integration tests
This PR is ready for review but it needs input-output-hk/catalyst-ci#138 to be merged first for the ci to pass

Related Issue(s)

List the issue numbers related to this pull request.

Closes #84

Description of Changes

Add a tests folder to be used for all tests except unit tests.
Add a test earthly target
Add integration test crate with a integration test example. This is just an example and willl be updated in a future PR
Add integration test earthly target. this is also an example, it will need to be updated with the cat-gateway docker image

Please confirm the following checks

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream module