Support proxy-header parameter when joining member clusters with karmadactl

[jwcesign]

What would you like to be added: Support proxy-header parameter when joining member clusters with karmadactl

Why is this needed: I have 3 member clusters, they can only be accessed through proxy with different header:

So when I want to use karmadactl to join these clusters through the proxy, I need to specify the headers.

So it‘s necessary to support proxy-header parameter when joining member clusters with karmadactl.

/cc @RainbowMango @GitHubxsy @XiShanYongYe-Chang

[jwcesign]

operator and helm need to be considered as well.

[XiShanYongYe-Chang]

/assign