feat(platform): impl restconfig provider

[leoryu]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind api-change
/kind bug
/kind cleanup
/kind deprecation
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[leoryu]

type K8sRestConfigProvider interface {
GetK8sRestConfig(ctx context.Context, client platforminternalclient.PlatformInterface, cluster *platform.Cluster, username string) (*rest.Config, error)
// remove this method in future
GetK8sRestConfigV1(ctx context.Context, client platformversionedclient.PlatformV1Interface, cluster *platformv1.Cluster, username string) (*rest.Config, error)
}

RestConfigProvider

GetRestConfig(