Fix token caching for multi-cluster multi-namespace environments

[dee-kryvenko]

See https://github.com/jenkinsci/hashicorp-vault-plugin/issues/260. New implementation uses caching criteria to differentiate tokens for different environments. So far I only came up with two possible attributes - server address and a namespace, but I have created CacheKey to allow for easy extension later on if more colliding attributes will be discovered. I have also added transient where I think it applies since I do not think we want to cache be ever written to the disk. I also added a little thread safety, although I am not sure if it is necessary (and I did not observe any issues with it so far) but it felt like it is.

• [x] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [x] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
• [x] Link to relevant issues in GitHub or Jira
• [x] Link to relevant pull requests, esp. upstream and downstream changes
• [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue