Add secrets handling abstraction

⚠ Blocked by !2288 (merged) ⚠

Before merging change the target branch to master

What does this MR do?

Implements basic abstraction for secrets resolving

Why was this MR needed?

Step three for #26565 (closed)

What's the best way to test this MR?

This is an intermediate step to add full Vault support implementation. If you'd like to see how the overall integration works in action, then:

1. Go to !2374 (merged) and build the binary based on that MR.
2. Set-up a development version of GitLab based on recent master (GDK or GCK should work for that)
3. Set-up a Vault server. I'm using this custom deployment on my local machine.
4. Clone the integration test project from: https://gitlab.com/tmaczukin-test-projects/test-secrets-integration.
5. Set-up GitLab and Vault accordingly to the test-secrets-integration README file and proceed with running a test job with your locally build Runner.

What are the relevant issue numbers?

Related to #26565 (closed)