juju register does not work for k8s controller

[Yang Kelvin Liu]

juju register does not support a k8s controller having `proxy-config`.

```
$ cat ~/.local/share/juju/controllers.yaml | yq '.controllers.k1.proxy-config'
config:
  api-host: https://127.0.0.1:16443
  ca-cert: |
    -----BEGIN CERTIFICATE-----
    MIIDDzCCAfegAwIBAgIUJMOPln3PfMts2ouNdSGXBiD2fzswDQYJKoZIhvcNAQEL
    BQAwFzEVMBMGA1UEAwwMMTAuMTUyLjE4My4xMB4XDTIyMTAyNzEyMzQwM1oXDTMy
    MTAyNDEyMzQwM1owFzEVMBMGA1UEAwwMMTAuMTUyLjE4My4xMIIBIjANBgkqhkiG
    9w0BAQEFAAOCAQ8AMIIBCgKCAQEAuO5FeUKN/7zCY6fERqAp0QxSR8ImasXqFlPI
    AD4SvVBb3YnGJwJPJrNVAVjhYcUASB3+HIJME4UsbfXSKKaizWm6oDxxHgeu+DIr
    vwiFk810xt8Ht4G48xOaYiSD0XghhOOu7MEPh2VU8sW/2KXULANzqRNy7pmcwdzZ
    SArh7AZ5bx5stQ4tPTkEKO+3gh07D3GNE7Jj+h7tPyxdfEQsqakpPyU9iWuahAM7
    pcQxTcqsTo5Hkogf4ZfmTvLlaOYVMSCghZB4UTm8pgWeWZObE8pyEjbgLWhLWCmw
    HTqi3s4nvXGsbkWq+sHqcEpsAo9v/fQYCJF53qc8wZZpl66p/QIDAQABo1MwUTAd
    BgNVHQ4EFgQUtZ5yTNMf46dF5zijl4kCIfQnp/MwHwYDVR0jBBgwFoAUtZ5yTNMf
    46dF5zijl4kCIfQnp/MwDwYDVR0TAQH/BAUwAwEB/zANBgkqhkiG9w0BAQsFAAOC
    AQEAB6aZk0ieKmqnC6Hr9H2y0FcVOC8L/785E/MCKB1KVpapQL3RgWBP/vNgR9RS
    BMoT3ZBW8BsBqcXL2gACJ3Vdm4xvC4Sx3ncE45Me7JbTlHjuJj1umeCDAT2B08KT
    iwRnmjC1ZUNqzMV0Ys/Z7DnNP8j8zxbJ+aPlpuZodi/1E5Z8jBbk7FHQVnOTNEJE
    Ec+YWzBWC2N6/0w5qopuTUkkmQNj2QjLCHfTjUy9LohBKg3hzUjvAfYm9T+OmzaR
    5UAyUfQQLTSJd7VRd7PpekrXSB9zoSRS/fpVQDDpL/PZmxgU9uGBi6aFTpW7Is+q
    HuWn0NUjyWvAmDs2CRJo+GkNbA==
    -----END CERTIFICATE-----
  namespace: controller-k1
  remote-port: "17070"
  service: controller-service
  service-account-token: eyJhbGciOiJSUzI1NiIsImtpZCI6ImxMeWVITWw2NzZ3SnhTM3pRUnZaQ1IwT2N3REhHT2FmNkkxRGdXWmUzR3cifQ.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.fBBt5cjOK6R7K7BMcAVQbmX18sVAhZpvxC3CGIj7nBc1mTr5X015wrRU4o33ERPC_-9_jGnlov8HRAfHkBBF8tys2H0bMF528Ip7Toxz7oy6lkO04Kz-_4XGykBXNgKZvuWqUR6rqbC7bzqSK7lbn2BUwD2szCJ6qhbWhm154csEoSvWmqW8zrO3nhzkawDeGpMqACOjeLxdxl2PXOUlPT47j37mYhEN8uOH6NgPbaldDO7c-icvKOmTC0Fe6xqIIAyloS8C7B2fmxBI171_LWHDedXcgJ94BYb072o9L3RcWhbvnT4fyqF0KDH1k6ZjyTAR1DBqSHVZ1hsoGhTnjw
type: kubernetes-port-forward
```

[Yang Kelvin Liu (kelvin.liu)]

https://github.com/juju/juju/pull/14878
https://github.com/juju/juju/pull/14892