apiserver: add enable subresource options

[Iceber]

What type of PR is this?

/kind feature

What this PR does / why we need it:
Enable support for the specified subresource via --allowed-proxy-subresources flags. No subresources are turned on by default

With --help you can see all the subresources that are supported to be turned on.

$ ./bin/apiserver --help
...

Resource server flags:

      --allowed-proxy-subresources strings
                List of subresources that support proxying requests to the specified cluster, formatted as '[resource/subresource],[subresource],...'. Supported
                proxy subresources include "pods/proxy,pods/log,pods/exec,pods/attach,pods/portfowrd,nodes/proxy,services/proxy"

...

example:

$ # Enable the proxy subresources for all resources and the exec subresources of the pods.
$ ./bin/apiserver --allowed-proxy-subresources "pods/exec,proxy"

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[clusterpedia-bot]

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