feat(helm): add volume mounts & cert vars

[ericpaulsen]

closes #9

this PR adds support for mounting volumes into the coder-logstream-kube container, and adds values for configuring the SSL_CERT_FILE and SSL_CERT_DIR envrionment variables.

since our customers typically leverage self-signed certificates, they need to add the Coder server certificate into coder-logstream-kube to establish the connection. here's an example values.yaml file that accomplishes this use-case:

volumes:
  - name: "my-volume"
    secret:
      secretName: cert-secret
volumeMounts:
  - name: "my-volume"
    mountPath: "/mnt/my-volume"
image:
  sslCertFile: "/mnt/my-volume/cert.crt"
  sslCertDir: "/mnt/my-volume"