Kubernetes Target support

**Is your feature request related to a problem? Please describe.**
> Boundary provides an easy-to-use, platform-agnostic way to access all of your hosts and services across clouds, Kubernetes clusters, and on-premises datacenters through a single workflow based on trusted identity. It lets you remove hard-coded credentials and firewall rules, and makes access control more dynamic.
~ https://www.hashicorp.com/blog/hashicorp-boundary

As per the above announcement, Kubernetes clusters as targets is either an existing or planned feature. Is there any guidance on how to use Boundary today for Kubernetes use-cases? If not, happy to contribute it as a target.

**Describe the solution you'd like**
I would like to login to any Kubernetes cluster that exists in a given Boundary project using an expiring service account for a limited amount of time with specified RBAC permissions to my user. I expect a new Kubernetes context to be added to either `export KUBECONFIG=` or to my `~/.kube/config` file.

**Describe alternatives you've considered**
Should I write my own target plugin?

**Explain any additional use-cases**
Apart from temporary access to a k8s cluster, it would be interesting to have the ability to request different RBAC permissions. For example, maybe I just want read-only access, and I can escalate my access on a need basis.

**Additional context**
Existing solutions focused on Kubernetes:
- [kconnect](https://github.com/fidelity/kconnect)