Allow Agent auto auth to read symlinked JWT files

[tomhjp]

This came up when helping someone to set up JWT auto auth with Vault Agent in Kubernetes. They had lots of Kubernetes clusters and so didn't want to have to set up a Kubernetes auth mount for each cluster, and instead wanted to use projected Service Account tokens with the JWT auth method. Kubernetes mounts projected tokens using symlinks from an immutable volume mount to help it rotate JWTs when they expire. In this case, the fact the JWT was a symlink stopped JWT auto auth from working, but I see no reason we can't support symlinks in this case.

Currently, the tests are a little fragile in relying on log output. I opted not to change the signature of ingressToken to return an error because the actual production code would never pay attention to or change its behaviour based on the return value, but I'm very open to feedback on that.

Lastly, I have opted to allow ioutil.ReadFile to handle any errors from cases such as a symlink pointing to a directory for simplicity, as when experimenting I found the error message was clear enough without additional handling, e.g. read sym-dir: is a directory. It's possible I've missed a separate motivation for the IsRegular() check though.