Add Hashicorp Vault secret resolver

⚠ Blocked by !2373 (merged) ⚠

Before merging change the target branch to master

What does this MR do?

Adds the Vault secrets resolver, that binds all parts together.

Why was this MR needed?

The fifth and final step for #26565 (closed)

What's the best way to test this MR?

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

1. Build the binary based on this MR or download it from the S3 development release.
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?

Closes #26565 (closed)