support loading client secrets from Kubernetes Secret Resource

[zhaohuabing]

This PR loads client secrets from Kubernetes Secrets when reading config.

A follow-up PR will be added to watch Kubernetes Secrets and update the config.

Fix #24

[codecov]

Codecov Report

Attention: Patch coverage is 91.11111% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 93.05%. Comparing base (27eba7b) to head (3e45ec5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
- Coverage   93.12%   93.05%   -0.07%     
==========================================
  Files          19       20       +1     
  Lines        1266     1311      +45     
==========================================
+ Hits         1179     1220      +41     
- Misses         51       55       +4     
  Partials       36       36              

Files	Coverage Δ
internal/secrets.go	91.11% <91.11%> (ø)

[nacx]

Added some comments. Thanks!

[nacx]

We will be loading other secrets, and I think we could decouple the K8s logic from the Config file itself, for a cleaner design. For example, we could:

• Create a SecretLoader in the internal package that receives the Config in the constructor and that implements the run.PreRunner interface. This will make its PreRun method be called after the configuration has been loaded.
• In the PreRun method, you can load the secrets and mutate the configuration accordingly.
• In the main.go, register the secretLoader in the run group immediately after the localConfig.

This way the k8s logic will be decoupled from the configuration, and if we need to reuse, say the k8s client in other services we'll already have the code prepared to share it more easily.

[nacx]

Added a small fix to the e2e tests, so that the kubeconfig is only loaded if there is any secret ref configured. It was trying to load it in the docker-based tests as well.
FYI: when e2e-tests fail, artifacts with all the logs are attached to the CI build, for further inspection.