helm: support custom client certs in nats-box

[wallyqs]

Make it possible to mount the certs for a sample client to test mutual TLS. something like:

natsbox:
  enabled: true
  tls:
    insecure: false
    verify: true
    secret:
      name: nats-client-tls
    ca: "ca.crt"
    cert: "tls.crt"
    key: "tls.key"

[kylos101]

Just making sure this doesn't fall off my radar.

[caleblloyd]

Supported in nats-1.0.0-beta.0