feat: Add Repository secret observation

[oskisk]

Description of your changes

Trigger a resource update on secret changes for all the references within the repository controller.

Fixes #36

I have:

• [x] Read and followed Crossplane's contribution process.
• [x] Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

• Added basic unit tests for the repository controllers
• manual testing for referenes and observations

[oskisk]

@MisterMX Would you have time to review this PR?

[oskisk]

Thanks for this PR @oskisk! I made some minor remarks. Can you take a look at them?

Thank you for reviewing. I've made the changes to the branch based on your suggestions.

[nikolainespor]

@MisterMX could you review the last changes from @oskisk ?