Allow matching also on pod names

As described in issue https://github.com/hashicorp/vault-plugin-auth-kubernetes/issues/65, a scoped token also includes the name of the pod it has been injected into.

What do you think about about adding an additional configuration parameter that would allow a vault auth role specification to allow matching not only on the kubernetes namespace and service account names, but also on the pod name (possibly using wildcards)?

This would allow to assign roles also based on the pod identity in all those cases when it is not possible to assign different service accounts to different pods, like when building (as in the referenced issue) with the gitlab runner that does assign the same service account to all the spawned build jobs.

If you think this is a reasonable feature to add, I can work to contribute a pull request.