gwctl: Introduce a `--for` flag to filter results related to a specified resource.

[gauravkghildiyal]

What type of PR is this? /kind feature

What this PR does / why we need it:

This PR achieves two broad things:

1. Add --for flag for filtering resources based on an associated resource.

   For example:

   • gwctl get backends --for gateway/ns2/my-gateway
   • gwctl get gateways --for httproute/ns1/my-httproute
   • gwctl get gateways --for gatewayclass/foo-gatewayclass
   • gwctl get policies --for service/foo-namespace/my-svc

2. Remove duplication from cli setup of Get and Describe commands. This inturn allows both commands to share feataures/flags (when applicable) without the need to manually configure them independently for each of them

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

gwctl allows filtering results to only those related to the specified resource (using the `--for` flag)

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gauravkghildiyal

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• ~~gwctl/OWNERS~~ [gauravkghildiyal]

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[gauravkghildiyal]

(Rebased)

[robscott]

Thanks @gauravkghildiyal!

/lgtm